Change many Parents' Evening appointments at once (administrators)

Requires permission: ParentsEvening

Cancels the given appointments, shifts them by a number of minutes (a teacher running behind or ahead), or moves those before a time to each teacher's next free slot after it (a family running late). A move goes only to a slot of the same session where the teacher is free and available and the family has nothing else; one that cannot is left where it is and says why. With preview, nothing changes and the plan comes back to be checked. Otherwise it is applied in one transaction and each family, and each teacher once they may see their bookings, gets one email listing their changes, as asked.