Extract a `ProtectedBranchAccessEe` module

Summary

• gitlab-org/gitlab-ee#1137 is a technical debt issue to clean up the EE protected branch access levels (for users and groups) implementation.
• Some of this cleanup bleeds over to code shared by CE and EE. This portion is covered in this CE merge request: gitlab-org/gitlab-ce!7821

References

• Closes #1137 (closed)

Tasks

• #1137 (closed) Follow-up from restricting pushes / merges by group
  • Implementation
    • Prefer validates with :uniqueness option!
      • https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/645#note_16390918
    • Could this be moved into ProtectedBranchAccess?
      • https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/645#note_16391013
      • https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/645#note_16391018
      • https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/645#note_16391023
    • Please name controller action specs after the method and action: describe "GET project_groups"
      • https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/645#note_16391093
    • I think we need more extensive integration specs here
      • https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/645#note_16391229
    • Does this need to be a proc?
      • https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/645#note_17145530
  • Tests
    • Added
    • Passing
  • Meta
    • CHANGELOG entry created
    • Documentation created/updated
    • API support added
    • Branch has no merge conflicts with master
    • Squashed related commits together
    • Check for clean merge with EE
    • Added screenshots
  • Review
    • Endboss
    • Miniboss
  • Wait for merge