Automatically remind people to RSVP #472

Just like we send follow-up emails for anyone that hasn't finished their application, we should send a follow-up email to those who haven't RSVP'd yet.

Loosely based on the following rules:

• Send follow-up 7 days after they were accepted (if signed up > 2 weeks before event)
• Send follow-up 5 days before event (if signed up > 10 days before event)
• Send follow-up 2 days before event (if signed up > 3 days before event)