1. Move to dropdowns instead of checkboxes. One each for "Allowed to
Push" and "Allowed to Merge"
2. Refactor the `ProtectedBranches` coffeescript class into
`ProtectedBranchesAccessSelect`.
3. Modify the backend to accept the new parameters.
1. Improve error handling while creating protected branches.
2. Modify coffeescript code so that the "Developers can *" checkboxes
send a '1' or '0' even when using AJAX. This lets us keep the backend
code simpler.
3. Use services for both creating and updating protected branches.
Destruction is taken care of with `dependent: :destroy`