User deletion

What about allowing organizers to only index and show users?

(But that won't close https://github.com/openSUSE/osem/issues/460 as no one is currently set as 'admin', thus no one can access/edit users)

We can either provide this to live instances via authorizing organizers to 'edit' any user they want (exclude destroy, which will be only for admins?), or manually set an admin (perhaps that's outside of the scope of this commit and should be done individually to the live instance, manually setting chosen user to be an admin?)