Merge branch 'artifacts' into 'master'

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

1 files changed, 4 insertions, 0 deletions

diff --git a/app/models/commit_status.rb b/app/models/commit_status.rb
index 7d54d83974a..d346c5d35d2 100644
--- a/app/models/commit_status.rb
+++ b/app/models/commit_status.rb
@@ -92,4 +92,8 @@ class CommitStatus < ActiveRecord::Base
   def show_warning?
     false
   end
+
+  def download_url
+    nil
+  end
 end