Merge branch 'ee-1137-follow-up-protected-branch-users-and-groups' into 'master'

CE-specific changes for gitlab-org/gitlab-ee#1137

## What does this MR do?

- 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, which is why this MR is required.
- An EE-specific MR has also been created: gitlab-org/gitlab-ee!927

See merge request !7821