Require at least one full/creator permission

I built this logic on the front end, but it might be a good idea to have it on the backend as well. 

Return an error if attempting to delete or demote a user's privileges if they are the last FULL or CREATOR for an event. There must be at least one user with FULL permissions.
This is somewhat nit picky, but it's an important distinction, the comment should read:
`If the conference has other full or ...`