Allow to define cache in `.gitlab-ci.yml`

This extends `.gitlab-ci.yml` syntax to allow specifying caching files and directories between builds, making it easy to preserve ex. gems.

```
cache:
  paths:
  - .bundle
  - vendor/

before_script:
- bundle install --path vendor/

rspec:
  script:
  - bundle exec rspec
```

This is based on Build Artifacts changes.

/cc@dzaporozhets 

See merge request !1786

Add flay: tool to find duplicate code
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

See merge request !1789

[ci skip]

Add method complexity check to CI
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

See merge request !1783

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Change "recent" scopes to sort by "id"

These scopes can just sort by the "id" column in descending order to
achieve the same result. An added benefit is being able to perform a
backwards index scan (depending on the rest of the final query) instead
of having to actually sort data.

See merge request !1788

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

These scopes can just sort by the "id" column in descending order to
achieve the same result. An added benefit is being able to perform a
backwards index scan (depending on the rest of the final query) instead
of having to actually sort data.

Prevent people from adding the link back.



See merge request !1938

Implement Build Artifacts

This implements #3028 

1. It stores artifacts in shared/artifacts,
1. It adds `artifacts` to `.gitlab-ci.yml`,
1. We use GitLab Workhorse to offload artifacts uploading,
1. To download artifacts it uses GitLab Workhorse X-Sendfile extension,
1. There's one "artifact" per-build. The new upload removes previous one and creates a new one,
1. Default max artifact size is set to 100MB - this can be changed in settings.

Missing things:
1. Support for `.gitlab-ci.yml`: `artifacts: true or git-ls-files` which will upload all non tracked files,
1. Artifacts passing between builds.

GitLab Workhorse changes: https://gitlab.com/gitlab-org/gitlab-workhorse/merge_requests/5
GitLab Runner changes: https://gitlab.com/gitlab-org/gitlab-ci-multi-runner/merge_requests/46

Syntax:
```
artifacts:
  untracked: true # default: false
  paths: # default: empty
  - bin/files
```


See merge request !1584

Send build name and stage in CI notification e-mail



See merge request !1776

Add allow_failure field to commit status API

Closes #3196

See merge request !1685

[ci skip]

Fix duplicate entry for 8.1.0 release

Fix Drone CI service template not saving properly

This fixes an Error 500 when attempting to save a Drone CI template in the service template page. `hook.url` was not being used and depended upon a project when none existed.

/cc: @bugagazavr

Closes #3419

See merge request !1774

Closes #3419

Remove text-rendering property

Continues !1779

Closes #3220

[ci skip]

See merge request !1784

Closes #3220

[ci skip]

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Replace CoffeeScript block into JavaScript in Views.

Remove CSS property preventing hard tabs from rendering in Chromium 45

This is to workaround a bug in Chromium 45 (https://code.google.com/p/chromium/issues/detail?id=446434),
which is the default browser in Ubuntu 14.04 and older.

Closes #3220

See merge request !1779

Remove duplicate CHANGELOG etnries

[ci skip]

See merge request !1782

[ci skip]

Bump rugged version to be in line with gitlab_git requirements



See merge request !1780

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

This is to workaround a bug in Chromium 45 (https://code.google.com/p/chromium/issues/detail?id=446434),
which is the default browser in Ubuntu 14.04 and older.

Closes #3220

Display target branch on MR list when it is different from project's default

Fixes #3072

The screenshot below shows both previous and current UI state:

![branch-mr-both](/uploads/5c1f0519e42bae4655f3eee37edfc356/branch-mr-both.png)

First merge request has default branch as a target, second one has non default branch as target.

See merge request !1741

Switch to state_machines-activerecord (Fixes: #3374)

state_machine is unmaintained

See merge request !1764