GitLab

Cycle Analytics: Events per stage

Adds list of events to each stage:

- Issue: list of issues created in the last XX days, that have been labeled or added to a milestone.
- Plan: list of commits that reference for the fist time an issue from the last stage.
- Code: list of MR created in this stage
- Test: List of unique builds triggered by the commits.
- Review: List of MR merged
- Staging: List of deployed builds
- Production: list of issues with the time from idea to production

Fixes #23449 

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !6859