Namespace license checks for Repository Mirrors

What does this MR do?

Add namespace license checks around Repository Mirrors (push & pull)

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

Q1: How to handle for future cases where this has already been set?

The user will not be able to configure the push/pull mirrors, and the mirroring will not run while no valid license is active.

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes gitlab-org/gitlab-ee#2577