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