Add staff with subusers

A subuser gets their own login with only the permissions you grant. Sharing your own password instead means anyone can cancel your service or wipe your files.

1. 1Open Users and invite by email address.
2. 2Tick only the permissions that person needs: console, files, schedules, backups.
3. 3Leave server deletion and billing off unless they genuinely need them.
4. 4Remove the subuser when they stop being staff.