Mirroring settings should be hidden/grayed when mirroring is disabled

Summary

Mirroring settings are available even when mirroring is disabled.

Steps to reproduce

Disable mirroring in Admin > Settings > Repository > Repository mirror by unchecking "Allow mirrors to be set up for projects". Then go to a project's mirroring settings in Settings > Repository > Mirroring repositories and see that you still have the settings. If you submit a change, it seems mirroring won't actually work.