Rename is_registered to is_approved
Restrict signups: non-staff users can only sign themselves up.
Registered users may not sign up again.
Merge branch 'restrict-signups' into 'dev'

Restrict signups further

Users can only sign themselves up and cannot access the signup page once they've submitted a signup request. Staff members may sign other users up. Added a signup link for admins to the navbar.

See merge request !4