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

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

Add allow_failure field to commit status API

Closes #3196

See merge request !1685

- Offloads uploading to GitLab Workhorse
- Use /authorize request for fast uploading
- Added backup recipes for artifacts
- Support download acceleration using X-Sendfile

Change documentation of converting a MySQL-Database to a Postgres one

The instructions were missleading. See -> https://gitlab.com/gitlab-org/gitlab-ce/issues/2904

closes #2904

See merge request !1549

[ci skip]