Configure temporal storage management for past reservations with "Confirmed" or "Pending" status

We should probably set up a system that deletes "successful" Reservations. When I say "successful", I mean that they were either Confirmed or Pending when the system clock passed their effective date.

Since I mentioned only "Confirmed" and "Pending" statuses, I'll give some context and explain my thought process. I think we'll probably want to retain cancelled reservations for some interval of time, just so that if an admin denies a booking request (or existing Confirmed booking) they can provide justification.

It'd probably be decently straightforward to allow admins to issue these justification messages, we could let them modify the "Additional notes" attribute for a Reservation. Then, we'd just need to expose that to the frontend, since we're only showing the "Purpose" and "Attendees" attributes to clients currently. This would be nice to have, just so that clients can understand the library administration's reasoning for their booking cancellation.