Disable ground control for avengers (non-admin staff)

[josh1248]

Avengers, which are classified as staff, currently have access to ground control:

The buttons to delete a whole assessment, for example, are active, and work (no actual submissions were harmed - this was on a test uploaded assessment). This would be quite dangerous as it means that 1 rogue avenger with ill intent can wipe out all submissions very quickly.

This check should be both a frontend and backend measure. Backend issue at: source-academy/backend#1162