1. It makes sense to reuse these constants since we had them duplicated
in the previous enum implementation. This also simplifies our
`check_access` implementation, because we can use
`project.team.max_member_access` directly.
2. Use `accepts_nested_attributes_for` to create push/merge access
levels. This was a bit fiddly to set up, but this simplifies our code
by quite a large amount. We can even get rid of
`ProtectedBranches::BaseService`.
3. Move API handling back into the API (previously in
`ProtectedBranches::BaseService#translate_api_params`.
4. The protected branch services now return a `ProtectedBranch` rather
than `true/false`.
5. Run `load_protected_branches` on-demand in the `create` action, to
prevent it being called unneccessarily.
6. "Masters" is pre-selected as the default option for "Allowed to Push"
and "Allowed to Merge".
7. These changes were based on a review from @rymai in !5081.
