Import volunteer/organizer RSVPs from Meetup.

New routes for looking at imported meetup data: /meetup_users and /meetup_users/:id

Meetup Users live in their own table: MeetupUser. Only their full name + meetup ID is stored.
In the future we could potentially store their profile photos from meetup.

rsvps now has a polymorphic association back to 'user', so an Rsvp could potentially be
for a regular (bridgetroll) User, or a MeetupUser. MeetupUser is configured to act like a user
who answered 'false' to all profile questions.

Because you can't use has_many :through polymorphically, the 'volunteers' and 'organizers'
associations on event still just return bridgetroll users. The 'legacy_volunteers' and
'legacy_organizers' associations will return MeetupUser records, and the methods
'volunteers_with_legacy' and 'organizers_with_legacy' add the two associations together.

The added complexity here is to support users adopting their MeetupUser RSVPs when
they sign up for Bridgetroll. We'll see if this technology holds up.

jquery datatables plugin is included for the benefit of sorting /meetup_users page.