Add RSVP form previews to organizer console

- If the person looking at the organizer console hasn't RSVP'd (so, like, an admin), submitting this form will make a new RSVP. :frowning: 
- Organizers already have an RSVP, so they'll just see a "user has been taken" error :neutral_face: 

The question is ... how messed up will things get if admins start randomly RSVPing to things, thinking they are in some kind of non-existent preview mode? Is there a lazy thing I could do? (Not show the buttons to admins?)