1. Reuse the same dropdown component that we used for updating these
   settings (`ProtectedBranchesAccessSelect`). Have it accept options
   for the parent container (so we can control the elements it sees) and
   whether or not to save changes via AJAX (we need this for update, but
   not create).

2. Change the "Developers" option to "Developers + Masters", which is
   clearer.

3. Remove `developers_can_push` and `developers_can_merge` from the
   model, since they're not needed anymore.