- EE-specific protected branch access level code lives in this new module, while
  CE or CE/EE code lives in the existing `ProtectedBranchAccess` module. This
  allows us to make changes without introducing conflicts.

- The access level model classes first include `ProtectedBranchAccess`, followed
  by `ProtectedBranchAccessEe`, which preserves the inheritance chain:

  {Push,Merge}AccessLevel > ProtectedBranchAccessEe > ProtectedBranchAccess