!581 has a lot of changes that would cause merge conflicts if not
properly backported to CE. This commit/MR serves as a better
foundation for gitlab-org/gitlab-ee!581.
= Changes =
1. Move from `has_one {merge,push}_access_level` to `has_many`, with the
`length` of the association limited to `1`. This is _effectively_ a
`has_one` association, but should cause less conflicts with EE, which
is set to `has_many`. This has a number of related changes in the
views, specs, and factories.
2. Make `gon` variable loading more consistent (with EE!581) in the
`ProtectedBranchesController`. Also use `::` to prefix the
`ProtectedBranches` services, because this is required in EE.
3. Extract a `ProtectedBranchAccess` concern from the two access level
models. This concern only has a single `humanize` method here, but
will have more methods in EE.
4. Add `form_errors` to the protected branches creation form. This is
not strictly required for EE compatibility, but was an oversight
nonetheless.
