Bulk booking change

* Bulk reschedule a whole session * Reschedule all of the bookings within that session * Option to add notes that are emailed with the reschedule email * Being able to email or text all bookings from one session at once would also be really useful (for example if we needed to warn people about a parking issue or road closure or something). We take up to 45 people per timeslot, and yesterday I had to change the times of 2 sessions - each booking had to be done manually, and to a custom time because the bookable ones were sold out. This took ages, and there is no way to add a note to customers, so we've had quite a few confused emails (I did email them all first but it seems most of them went to spam so they didn't see the explanation).