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
- Organizers already have an RSVP, so they'll just see a "user has been taken" error
Merge branch 'master' into lillie-organizer-rsvp-form-previews
Consolidate volunteer and student rsvp preview routes

Remove 'submit' button from RSVP when previewing