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.
