Put a detached or cancelled Occurrence back on its repeating schedule

Clear the Occurrence's detached state and its cancellation, and restore the times the rule currently gives its seriesDate and Shift; its registrations and Day Attendance return to that date's day (unless that day is cancelled). Takes no body. Notifies nobody. The slot must belong to a Series: a one-off slot is 400 bad_request. An Occurrence whose Shift the rule no longer has is 400 bad_request. Reconnecting an Occurrence that is not detached is 200 and re-applies the rule's times (idempotent).