Add button to delete all merged branches
## What does this MR do?
It adds a button to the branches page that the user can use to delete all the branches that are already merged. This can be used to clean up all the branches that were forgotten to delete while merging MRs.
**Note**
~~This MR is WIP until MR !6408 is merged.~~
## Are there points in the code the reviewer needs to double check?
The UX of the actual "Delete merged branches" button.
## Why was this MR needed?
Fixes#21076
## Screenshots
Before:
After:
## Does this MR meet the acceptance criteria?
- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [x] API support added
- 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 you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)
## What are the relevant issue numbers?
Closes#21076
See merge request !6449
374b8e95c3