Notification endpoints does not process bitfields

[tsa96]

Old backend's method of doing updates to notifyOn values on MapNotify and is to batch all the flags into a bitfield where each ActivityType value x corresponds to the a 1 << x bit shift. It then stores that bitfield in DB, and genNotifications handles breaking down the field into flags.

This method seems fine to me, though we'll need to use a raw query for the DB stuff, since Prisma still doesn't support bitwise operations and given their bizarre prioritization decisions, probably won't for a long time. Maybe possible with Prisma Client Extensions? Don't really mind doing the raw query either way.

Actually supporting this is pretty much just a matter of changing the validators on relevant endpoints to allow values that aren't individual ActivityTypes, and do #760 so notifications actually get generated.

Ideally write a bunch more tests for notification-related behaviour. The old API had a couple of really complex E2E tests, we could do similar, e.g. submitting a map, it getting approved, someone following it, then someone submitting a run on it, checking it generates notifications. Could do as backend E2E, though if we wanna get really fancy, we could do in frontend E2E - honestly this sounds like the way to go.