Artifacts expire date

What do you think @grzesiek?

The syntax will be simple:

job:
  artifacts:
    expire_in: 7d

• Implement expire_in
• Check current design of expiry information with @jschatz1 and @markpundsack
• Add tests in GitLab application for a ExpireBuildArtifactsWorker and for ArtifactsController::keep
• Add user documentation how to use artifacts:expire_in
• Prepare GitLab Runner changes to pass expire_in: gitlab-org/gitlab-ci-multi-runner!191
• Fix timeago with help of @jschatz1
• Merge latest master after builds view changes @iamphill
• Add Omnibus support for expire_build_artifacts_worker cron job https://gitlab.com/gitlab-org/omnibus-gitlab/merge_requests/821
• Add documentation how to configure expire_build_artifacts_worker: https://gitlab.com/gitlab-org/gitlab-ce/issues/18694

This is based on https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4201.