Fix ordering issues with student waitlist rsvps

* student_waitlist_rsvps is now always ordered by waitlist_position
* waitlist on the Event Show page shows the real waitlist_position
instead of the fetched rsvp index (though they should now be the same)
* Reload event in-between destroying an rsvp and reordering waitlist,
otherwise the student_rsvps_count isn't reloaded and no-one is promoted
* Don't use find_each in reorder_waitlist!, because it
apparently doesn't respect order in Rails 4.1