Renamed $post_id to more descriptive $attendee_id.
Rename camptix_require_login_confirm_username to camptix_rl_registration_confirmed.

The camptix_rl_registration_confirmed hook was being used when the buyer completed their registration and when the
additional attendees confirmed their registration, but these are actually two distinct events and should trigger
separate actions.
Fire camptix_rl_buyer_completed_registration when buyer completes transaction.