Allow group members to RSVP to an Event

[loicknuchel]

Group member should be able to RSVP to an Event. They should be able to say Yes or No.
Number of Yes RSVP should be lower than the max attendee count of the Event.
Additional RSVPs should be taken in account but as a Waiting status (Yes but not accepted still).

RSVP order is very important, it can be kept using the RSVP time.

If a RSVP is changed from Yes to No, the older Waiting RSVP should be moved to Yes and the user notified by mail of this change.