Add a per-event setting to toggle RSVP support #162
Description
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