Implement special GitLab markdown reference for milestones

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3453 by @eReGeBe, with two additions:

- Move changelog item to 8.8
- Fix cross-project milestone ref with invalid project, like https://gitlab.com/gitlab-org/gitlab-ce/commit/f7348cd348ad8f4a18d74dd668283a4e236f5790 did for labels

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/13829

See merge request !3897

Fix group visibility level migration in case all visibility levels are restricted

fixes #14632 

See merge request !4107

Let users set notification levels in projects which they are not members

Fixes #9013 

See merge request !3986

Fix Error 500 when attempting to retrieve project license when HEAD points to non-existent ref

Closes #17537

See merge request !4151

Use extended regexp flag for `git grep`-ing files

Closes #3055

See merge request !4203

Update CE code to include some refactor done in EE to do with import url

This is simply updating the code to match EE and avoid further conflicts related to `import_data` and `import_url` changes made on EE only.

See merge request !4223

Updated the DB schema

This updates `db/schema.rb` which apparently wasn't fully updated in the recent merge requests.

See merge request !4222

Add config for CI Runner that prevents it from picking untagged jobs

Closes #3456

See merge request !4039

Fix creation of Ci::Commit object which can lead to pending, failed in some scenarios

## What does this MR do?

If we use a new `project.ci_commits` it will add it to array, and some other services which can do a save on a project can lead to a scenario when `ci_commit` will be saved, where it should not be.

## What are the relevant issue numbers?

https://gitlab.com/gitlab-com/support-forum/issues/717 https://gitlab.com/gitlab-com/support-forum/issues/715 https://gitlab.com/gitlab-org/gitlab-ce/issues/17596 https://gitlab.com/gitlab-com/support-forum/issues/714 https://gitlab.com/gitlab-org/gitlab-ce/issues/13402

cc @rymai 


See merge request !4214

Move generator templates to generator_templates/



See merge request !4217

Create a todo on failing MR build

Implements #14067. I worked on this with @DouweM (any mistakes are mine).

When a build fails for a commit, create a todo for the author of the merge request that commit is the HEAD of. If the commit isn't the HEAD commit of any MR, don't do anything. If there already is a todo for that user and MR, don't do anything.

Current limitations:
- This isn't configurable by project.
- The author of a merge request might not be the person who pushed the breaking commit.
- I haven't tested this with a working CI setup, just with the unit tests below and by modifying my DB directly.


See merge request !3177

Syntax-highlight diffs in push emails

![image](/uploads/8ecbabc65382214b8de63aae24f66cea/image.png)

Based on:
https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/151



See merge request !4147

Update 'after_script' introduction note

Related to gitlab-org/gitlab-ci-multi-runner#1321

See merge request !4205

Add pipeline view

This is continuation of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3653

cc @DouweM @grzesiek 

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/17551
Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/15625


See merge request !3703

Removed outdated comment from migration helpers

[ci skip]

See merge request !4213