Allow setting "Allowed To Push/Merge" while creating a protected branch.
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.
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