Prevent superusers

[rixx]

Superusers are granted more rights than they should be – they see buttons to accept a submission when it's already been accepted, for instance, since permission checks kinda short out. We should either fix this, or – and this may be more desirable anyways – introduce an admin role that is permitted to create new events, and stop relying on is_superuser completely, since being a superuser has weird connotations, and an admin or is_staff is clearer. We should also show a large warning when a superuser is using the site.