Open tournament registration

Moves an APPROVED tournament to REGISTRATION_OPEN. Takes no request body. Registration can only be opened from DRAFT or REGISTRATION_CLOSED; any other status answers 400. A tournament that is not APPROVED answers 403 error_approval_required. Side effects worth knowing about: a PUBLIC tournament becomes publicly listed, a tournament.registration_opened webhook is emitted, and when check-in is required and a start time is set the check-in announcement is scheduled. Requires an API key holding the tournaments lifecycle capability whose owner is a tournament ADMIN - a tournamentStaff MODERATOR is NOT sufficient and receives 403.