Remove destructive GET request from RSVP workflow

After some discussion, and some research into how other sites deal with carrying intentions through a login cycle I decided to remove the special case, destructive GET request for RSVPing to events. This simplifies the code and makes RSVPing to event work exactly like following groups.
@kdeloach I think it will unless you `prefetch_related('group')` or `select_related('group')` when retrieving the Events