CiviEvent: Batch participant status change screen

CiviEvent admins should be able to see all of the participants who are on a waitlist for a given event, as well as all that are awaiting approval - in general, participants sharing a particular status.

The admin should be also able to mass-update selected particpants to other status types (use case: approving participants that await approval, which moves them from Awaiting Approval to Pending from Approval).

For this, extend the current 'batch update from profile' functionality and base it on a reserved profile (and for this, make UF fields and groups to be reservable, and the groups to be addressable by name from the PHP side).