Deprecate all callers of `Event::getRegistrations()`

[oliverklee]

• [ ] Event.attachRegistration: only used for tests
• [ ] Event.getAttendeeNames: never called except for tests
• [ ] Event.getQueueRegistrations:
  • [ ] called by Event.hasQueueRegistrations, which is never called except for tests
• [ ] Event.getRegistrationsAfterLastDigest:
  • [ ] called by Event.getAttendeeNamesAfterLastDigest, which is never called except for tests
• [ ] Event.getRegularRegistrations,
  • [ ] which is called by Event.getRegisteredSeats,
  • [ ] which is called by Event.getVacancies
    • [ ] which is called by Event.hasVacancies
    • [ ] which is called by Event.isFull, which is never called except for tests
  • [ ] and Event.hasEnoughRegistrations,
    • [ ] which is called by EventStatusService.updateStatusAndSave
• [ ] EmailService.sendEmailToAttendees

[oliverklee]

Superseded by a handful of smaller tickets. Closing.