add volunteer interest to rsvp

[cjmanetta]

This PR adds a volunteer interest checkbox to the Rsvp form and migrates the Rsvp table to contain a volunteer interest boolean. This can be viewed in the organizer attendee details panel.
#443
PTAL

[lilliealbert]

@cjmanetta Thank you for this! Overall it looks really good. @tjgrathwell and I were chatting about whether it makes more sense for volunteer interest to live on the RSVP model, or if there should be some kind of join table between chapters & users like ChapterVolunteerInterest or something.

The main concern is that we'd like to collect this info when people RSVP, but it isn't really related to the RSVP itself — when we show the responses, we'll probably do it on a per-chapter basis, aggregating all the people who want to help out, and thinking about that query highlights for me how it'd be a little weird to get the info off of RSVP.

What do you think? Do you think a join table would be reasonable, or do you see another way we could model this? 😸