Award emoji supports retina displays

Based on discussion in !2800

There is now a retina spritesheet for emojis. It is 2mb, don't know how big an issue that is.

Before:

![Screen_Shot_2016-02-19_at_11.07.25](/uploads/ea412579bcaddfd13fce67bf583de61b/Screen_Shot_2016-02-19_at_11.07.25.png)

After:

![Screen_Shot_2016-02-19_at_11.07.36](/uploads/9d44c6278ba31987bad03dc1f46edd8b/Screen_Shot_2016-02-19_at_11.07.36.png)

Also updated the rake task to make the @2x spritesheet https://gitlab.com/snippets/15348

cc\ @rspeicher @vsizov 

See merge request !2891

Flush emptiness caches whenever needed



See merge request !2903

This ensures that the emptiness cache (used for Repository#empty? and
Repository#has_visible_content?) is flushed after comitting changes
(using the web editor, API or Git) for new repositories. Once a
repository is no longer empty there's no need to explicitly flush the
cache for Repository#empty?. The cache for
Repository#has_visible_content? in turn is already flushed whenever
needed.

Fixes gitlab-org/gitlab-ce#13387

Fix docs on artifacts API

Small fixes for https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2893

- Backticks in parameters
- Remove headings
- Fix curl URL link



See merge request !2902

- Backticks in parameters
- Remove headings
- Fix curl URL link

[ci skip]

Move a few controller specs to their correct locations



See merge request !2889

Add a `Blob` model that wraps `Gitlab::Git::Blob`

This allows us to take advantage of Rails' `to_partial_path` to render
the correct partial based on the Blob type, rather than cluttering the
view with conditionals.

It also allows (and will allow in the future) better encapsulation for
Blob-related logic which makes sense for our Rails app but might not
make as much sense for the core `gitlab_git` library, such as detecting
if the blob is an SVG.

See merge request !2887

Improve readability of 'rake cache:clear' code



See merge request !2892

Introduce API for serving the artifacts archive

Add API to download build artifacts archive in context of GitLab API

/cc @DouweM @grzesiek 

See merge request !2893

Add documentation about build emails service

Closes #12497

See merge request !2871

[ci skip]

Labels should be visible in milestone view

Closes #3276 

See merge request !2599

Fix missing ignored? for GenericCommitStatus

This is pretty important fix. We should not release 8.5 without this.

It critical error when handling GenericCommitStatus:
```
undefined method `ignored?' for #<GenericCommitStatus:0x007fe1a2a20fe0>
```

cc @grzesiek 


See merge request !2899

revert back to ruby 2.1

- - -

Revert Ruby 2.2 changes - until we solve https://gitlab.com/gitlab-org/gitlab-ce/issues/13514

This also updates Ruby 2.1.7 to 2.1.8

See merge request !2898

Add ability to revert changes introduced by Merge Requests or Commits

Closes #3409 

See merge request !1990

Consistency in MR and issue header.

Fixes: #13550 

![Screen_Shot_2016-02-18_at_11.20.19_AM](/uploads/8327ff045e45029b85b2cbac72c98a7f/Screen_Shot_2016-02-18_at_11.20.19_AM.png)![Screen_Shot_2016-02-18_at_11.20.55_AM](/uploads/f79613ebb23e4a62fb1bdb48d6f7fd2b/Screen_Shot_2016-02-18_at_11.20.55_AM.png)

See merge request !2874

I forgot to update this and some spinach specs were failing.

[ci skip]

* Now it's only created when user choose to create a MR and
  logic to hide it is handled by https://gitlab.com/gitlab-org/gitlab-ce/blob/50595af7b0e304f16e2955109abd2d25cd96efda/app/helpers/application_helper.rb#L139