[EE port] Add QA test for authorized push to protected branch

What does this MR do?

Ports https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18696 to EE.

Are there points in the code the reviewer needs to double check?

In particular, this MR adds a missing qa-allowed-to-push class to an EE-only view which has a lot of duplication with a CE view.

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a Backend maintainer
• EE specific content should be in the top level /ee folder
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?