Add a per-event setting to toggle RSVP support

[aaronpk]

Some events don't need to support RSVPs, such as deadlines and anniversaries.

Individual events should have a checkbox to toggle RSVP support.

Since the whole site has the option of enabling RSVPs or not, that means:

• Site RSVPs disabled:
  • No checkbox to turn on RSVPs for events
• Site RSVPs enabled:
  • Checkbox to turn on RSVPs for events, default checked