Standardize the way we check for and display form errors

- Some views had a "Close" button. We've removed this, because we don't
  want users accidentally hiding the validation errors and not knowing
  what needs to be fixed.
- Some views used `li`, some used `p`, some used `span`. We've
  standardized on `li`.
- Some views only showed the first error. We've standardized on showing
  all of them.
- Some views added an `#error_explanation` div, which we've made
  standard.

See merge request !3531

First pass at a Code Review guide

Largely borrowed from thoughtbot's code review guide, so attribution is
included.

[ci skip]

See merge request !3639

Updated UI for new merge request

Closes #2540 

![Screen_Shot_2016-03-29_at_12.53.18](/uploads/426e603675171f0dc4e0af83c7762eba/Screen_Shot_2016-03-29_at_12.53.18.png)

![Screen_Shot_2016-03-29_at_12.53.13](/uploads/645ad7955acfb9f7693245fcc048ee49/Screen_Shot_2016-03-29_at_12.53.13.png)

See merge request !3228

Remove `random_markdown_tip` helper

Only usage got removed in !3442

See merge request !3637

Correct documentation on enabling artifacts.

The docs say to enable artifacts that settings should be set to "false".  This updates the documents to change the setting to "true".

See merge request !3630

Removed important from css

Use false/true mixin in migration from !3640



See merge request !3646

Fix more SQL migrations to use raw commands

gitlab-org/gitlab-development-kit#109

See merge request !3640

Upgrade annotate from 2.6.10 to 2.7.0

We don’t have any tests for this, so no need to run them. [ci skip]

Includes compatibility with Rails 5.

Changelog: https://github.com/ctran/annotate_models/releases/tag/v2.7.0

Working towards #14286.

See merge request !3645

[ci skip]

Upgrade premailer-rails from 1.9.0 to 1.9.2

Includes compatibility with Rails 5.

Changelog:
https://github.com/fphilipe/premailer-rails/blob/master/CHANGELOG.md#v192

Working towards #14286.

See merge request !3643

Fix seed_fu failure with inserting milestones into test DB

Milestones were not being saved due to "invalid state" validation errors

See merge request !3641

Ignore toggling the sidebar for resolutions over 1200px width

Closes #14353 
![sidebar1](/uploads/4b8d0eaa1b320c319239cb572932f740/sidebar1.gif)

See merge request !3547

prevent users from being able to both upvote and downvote



See merge request !3495

This happens because tab's `shown` event is triggered first if we enter directly to the diff page therefore Breakpoints class is not initialized yet.

gitlab-org/gitlab-development-kit#109

Includes compatibility with Rails 5. We don’t have any tests for this,
so no need to run them. [ci skip]

Changelog: https://github.com/ctran/annotate_models/releases/tag/v2.7.0

Working towards #14286.

Includes compatibility with Rails 5.

Changelog:
https://github.com/fphilipe/premailer-rails/blob/master/CHANGELOG.md#v19
2

Working towards #14286.

Remove tab stop from "Write", "Preview", "Go full screen" links

Closes #15089

See merge request !3635

Redesign labels page

closes #14227 

## Desktop
![Screen_Shot_2016-04-05_at_7.19.37_PM](/uploads/a52b764565b6e2739fb6932178f8bde5/Screen_Shot_2016-04-05_at_7.19.37_PM.png)

## Mobile
![Screen_Shot_2016-04-04_at_6.40.57_PM](/uploads/9804b074126fcdb6755918e9f4c472a7/Screen_Shot_2016-04-04_at_6.40.57_PM.png)

See merge request !3507

Milestones were not being saved due to "invalid state" validation errors

Everything on one page.



See merge request !3636

Use raw SQL instead of Rails models in 20130403003950 migration

Closes gitlab-org/gitlab-development-kit#109

Closes https://github.com/gitlabhq/gitlabhq/issues/10123

See merge request !3638

Largely borrowed from thoughtbot's code review guide, so attribution is
included.

[ci skip]

Closes gitlab-org/gitlab-development-kit#109

Closes https://github.com/gitlabhq/gitlabhq/issues/10123

Only usage got removed in !3442

Add a `sidekiq` tag to Sentry Raven context

This will let us filter errors that were/were not in the Sidekiq
environment.

Source: http://stackoverflow.com/a/28370539/223897

Closes #15092

See merge request !3634

Closes #15089

This will let us filter errors by the program environment in which they
were encountered.

Source: http://stackoverflow.com/a/28370539/223897

Closes #15092