API: Expose 'developers_can_push' for branches
## What does this MR do?
Adds support for the `developers_can_push` flag for the branches API. It also supports creating protected branches with that flag.
## Are there points in the code the reviewer needs to double check?
The API call requires an optional boolean parameter `developers_can_push`. If it is not either `true` or `false`, it should not update the value. I'm not sure if this is the right way to do it.
Right now it always returns `200`. Maybe the `PUT` method should return different status codes depending if it really created or updated a branch.
## What are the relevant issue numbers?
Closes#12735
## 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 [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)
See merge request !5208
d6bd412be4