allow setting course to "None" which will then omit teaching-related UI on volunteer sign-up.  Also added some text and grouped teaching/student related items on event form in the hopes of making it more clear.
remove debugging cruft -- oops!
teaching experience should only be required if a volunteer selects "Teaching" or "TAing" -- adjusted order of fields on form and added some help text to make that more clear
Merge branch 'volunteer_event' of https://github.com/ultrasaurus/bridge_troll into ultrasaurus-volunteer_event
Use nested with_options calls to DRY up event/rsvp validations
Ask organizer whether events will have "Students and Volunteers" or "Just Volunteers"

This is instead of the confusingly-purposed "Allow student rsvp?" checkbox.

When an organizer selects "Just Volunteers", it hides fields that are only relevant for
student events: 'Course', 'student rsvp limit' and 'student details'.

Also many other assorted changes to the event show page so that volunteer-only
events don't say "student" all over the place.