Use optimistic locking

## What does this MR do?
Removes the usage of pessimistic locking in favor of optimistic which is way cheaper and doesn't block database operation.

Since this is very simple change it should be safe. If we receive `StaleObjectError` message we will reload object a retry operations in lock.

However, I still believe that we need this one: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7005 as this will reduce a load on Database and FS.
This changes a behavior from:

### Pesimistic locking (previous behavior)

#### For updating
1. SELECT * FOR UPDATE (other updates wait on this)
2. we update ci_pipeline
3. latest_build_status
4. enqueue: (use: transition :created -> :pending)
5. [state_machine] we are in  state created, we can go to pending
6. [state_machine] ci_pipeline.status = created
7. [state_machine] ci_pipeline.save
8. [state_machine] after_transition: (if for success): PipelineSuccessWorker on Sidekiq
9. release DB lock

#### If no update is required
1. SELECT * FOR UPDATE (other updates wait on this)
2. we update ci_pipeline
3. latest_build_status
4. we are in pending, we can't transition to pending, because it's forbidden
5. release DB lock

### Optimistic locking (implemented by this MR)

#### For updating
1. latest_build_status
2. enqueue: (use `transition :created -> :pending`)
3. [state_machine] we are in state created, we can go to pending
4. [state_machine] ci_pipeline.status = created
5. [state_machine] ci_pipeline.save
6. [state_machine] [save] where(lock_version: ci_pipeline.lock_version).update_all(status: :created, updated_at: Time.now)
7. [state_machine] [save] unless we_updated_row then raise ObjectInconsistentError

#### If no update is required
1. we update ci_pipeline
2. latest_build_status
3. we are in pending, we can't transition to pending, because it's forbidden

## Why was this MR needed?
We have been seeing a number of problems when we migrated Pipeline/Build processing to Sidekiq. Especially we started seeing a lot of blocking queries.

We used a pessimistic locking which doesn't seem to be required. This effectively allows us to fix our issues with blocked queries by using more efficient method of operation.

## What are the relevant issue numbers?
Issues: https://gitlab.com/gitlab-com/infrastructure/issues/623 and https://gitlab.com/gitlab-com/infrastructure/issues/584, but also there's a bunch of Merge Requests that try to improve behavior of scheduled jobs.

cc @pcarranza @yorickpeterse @stanhu

See merge request !7040

Delete issue board welcome cookie

## What does this MR do?

This was seen in the idea to production demo, because the project had already existed on the exact same URL the cookie already existed which meant the blank state wasn't shown. 

This MR deletes the cookie on project create.

See merge request !6899

Replace static fixture by generated one in issue_spec.js

## What does this MR do?

- clean up `issue_spec.js`
- introduce an alternative approach to #19445
  - rename `rake teaspoon` to `rake teaspoon:tests`
  - introduce `rake teaspoon:fixtures` which generates fixtures using RSpec
  - introduce `rake teaspoon` which runs `rake teaspoon:fixtures` and `rake teaspoon:tests`

## Why was this MR needed?

- many duplications
- missing existence checks
- missing conditions
- static fixtures don't match real views

## Reasoning

I want to explain some of my decisions here, so that they stay visible for future discussions.

### Why not HAML?

- same number of HAML templates as number of fixtures (many input files)
- embedded logic less readable
- can not be rendered by JavaScript (because of inline Ruby)

### Why RSpec?

- real controllers for fixtures
- spys available for mocking
- easily report failed fixture generations

### Why not magic_lamp? (#19445)

- introduces another dependency/tool
- needs to run a server concurrently to teaspoon
- makes it harder to use a JavaScript test runner
- static HTML files serve faster

See merge request !6059

Modify GitHub importer to be retryable

Closes #23533

See merge request !7003

Only show one error message for an invalid email

Changes it to only validate the notification_email format if it's different from email.

Closes #5905

See merge request !7158

Stop unauthorized users dragging on milestone page

Closes #13670

See merge request !7113

Document admin access requirement for /deploy_keys endpoint

Closes #23807

See merge request !7164

Fix and improve `Sortable.highest_label_priority`

Closes #23928

See merge request !7165

API: Fix booleans not recognized as such when using the `to_boolean` helper

Fixes #22831
Fixes #23890

See merge request !7149

Jira refactoring to jira gem

part of gitlab-org/gitlab-ee#1008

Original MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2572/commits

See merge request !6598

Prevent a spec from failing in `spec/features/merge_requests/diff_notes_resolve_spec.rb`

## What does this MR do?

Fixes a failing spec in `spec/features/merge_requests/diff_notes_resolve_spec.rb`

## Are there points in the code the reviewer needs to double check?

## Why was this MR needed?

Tests failing on master. RSpec executes inner `before` blocks after outer `before` blocks, so the second note was being added *after* loading the page, and then being pulled in by running JS on the page, causing intermittent failures if we clicked the resolve button before the note was loaded.

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG.md) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

Closes #23875

See merge request !7150

Merge branch 'bugfix/dragging_milestones' of gitlab.com:blackst0ne/gitlab-ce into bugfix/dragging_milestones

Fix couple of GitHub importing bugs

Fix a bug in GH comment importing and label applying for imported MRs.

See merge request !7139

edited stylesheet with mr modal fixes and added to changelog

## What does this MR do?
It shortens the modal command merge info elements.

## Are there points in the code the reviewer needs to double check?
I used the `.dark` class.. is that correct usage?
Made no issue as this is a minor upgrade to UI

## Why was this MR needed?
Clipboard buttons could obscure information in the merge info modal, this way its clean.

## Screenshots (if relevant)

From:
![image](/uploads/99269c9a314c593cb06239c27565b07c/image.png)

To:
![image](/uploads/7e278a5aaa8c57270e8a320cc270b5d8/image.png)


## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?
None, its a quick fix

See merge request !7145

Add section about contributing to README

Closes #23830

See merge request !7137

Get fix for teaspoon tests

Replaced jquery.cookie with js.cookie

## What does this MR do?

Fixes currently failing `teaspoon` build by replacing the last reference to `jquery.cookie` with `js.cookie`.

## Are there points in the code the reviewer needs to double check?

## Why was this MR needed?

`master` failing.

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG.md) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

See merge request !7160

Getting latest CHANGELOG so I can resolve merge conflict

If notification_email is blank, it's set from email. If an admin
attempted to create a user with an invalid email, an error would be
displayed for both fields. Only validate the notification_email if it's
different from email.

Enable linting for ES6 files

## What does this MR do?

Add `.js.es6` to file extensions checked by ESLint.

## What are the relevant issue numbers?

Follows !5445

See merge request !7156

Document CI pipelines settings

See merge request !5847