CiviEvent: Waitlisting

Introduce CiviEvent waitlisting.

If an event has waitlisting enabled and the event gets full, instead of redirecting the users to a 'sorry, the event is full' notification the users should be shown a screen with the information that the event is currently full, but that they can register temporarily and be put on a waitlist.

Once a place on the event opens (triggered by the event limit changed, or a participant who was previously counted against the event limit being no longer counted against it - either because they canceled their participation, didn't confirm their participation in time, got rejected by the event admin or for any other reason), the first person from the waitlist - based on civicrm_participant.register_date - should be (a) put in a 'Pending from waitlist' state and (b) notified via email that they should return to the event completion screen, where they confirm their participation (and pay if the event is paid).

When a participant moves from On waitlist to Pending from waitlist, both the participant and the civicrm_event.(b)cc_confirm address(es) should be mailed about the change, with the link to click to complete the registration.