Artifacts expire date

What do you think @grzesiek?

The syntax will be simple:
```
job:
  artifacts:
    expire_in: 7d
```

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

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

See merge request !4200