* Now it's only created when user choose to create a MR and
  logic to hide it is handled by https://gitlab.com/gitlab-org/gitlab-ce/blob/50595af7b0e304f16e2955109abd2d25cd96efda/app/helpers/application_helper.rb#L139

* Now checking if a commit is already reverted or there is a conflict is
  much more faster.
* No longer need to create a new branch.

* Not required to run hooks since it's an internal commit

Make it possible to erase build content (artifacts, trace)

This feature makes it possible to remove build content - build artifacts and build traces.

- [x] Remove artifacts
- [x] Remove metadata
- [x] Remove build traces
- [x] Wait for https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/1942 this to be merged
- [x] Fix the permissions after the merge

Closes #3421

See merge request !2560