Implement Build Artifacts

This implements #3028 (closed)

1. It stores artifacts in shared/artifacts,
2. It adds artifacts to .gitlab-ci.yml,
3. We use GitLab Workhorse to offload artifacts uploading,
4. To download artifacts it uses GitLab Workhorse X-Sendfile extension,
5. There's one "artifact" per-build. The new upload removes previous one and creates a new one,
6. 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,
2. 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

app/controllers/admin/application_settings_controller.rb

@@ -58,6 +58,7 @@ class Admin::ApplicationSettingsController < Admin::ApplicationController
:admin_notification_email,
:user_oauth_applications,
:shared_runners_enabled,
+      :max_artifacts_size,
restricted_visibility_levels: [],
import_sources: []
)