Fix bugs in MergeWhenSucceeded

1. This fixes support for merge when succeeded for statuses without ref.
2. This fixes support for merge when succeeded for multiple stages. Stages are created after all builds for previous one are finished.

Fixes: https://gitlab.com/gitlab-org/gitlab-ce/issues/9060 https://gitlab.com/gitlab-org/gitlab-ce/issues/8108 https://gitlab.com/gitlab-org/gitlab-ce/issues/12931 https://gitlab.com/gitlab-org/gitlab-ce/issues/13269

/cc @grzesiek @DouweM @rspeicher 


See merge request !2894

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

Use around_transition to trigger build creation for next stages

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]