1. `GitPushService` was still using `{merge,push}_access_level_attributes` instead
   of `{merge,push}_access_levels_attributes`.

2. The branches API creates access levels regardless of the state of the
   `developers_can_{push,merge}` parameters. This is in line with the UI,
   where Master access is the default for a new protected branch.

3. Use `after(:build)` to create access levels in the
   `protected_branches` factory, so that `factories_spec` passes. It
   only builds records, so we need to create access levels on `build` as
   well.

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.

1. The new data model moves from `developers_can_{push,merge}` to
   `allowed_to_{push,merge}`.

2. The API interface has not been changed. It still accepts
   `developers_can_push` and `developers_can_merge` as options. These
   attributes are inferred from the new data model.

3. Modify the protected branch create/update services to translate from
   the API interface to our current data model.

not_found appends string "Not Found" to the argument causing the resulting message to be "Branch does not exist Not Found" which is an incorrect error message.

Changed the argument of not_found! for 'unprotect'  command to "Branch" from "Branch does not exist".  This makes the final error message to appear as "Branch Not Found" which is correct and same as error messages for other commands like 'protect'.