Feature: delete all signups of an event

[samporapeli]

Signups can be deleted, but currently only one at a time. It would be convenient to remove all signups of a event.

Currently there's a feature that tries to anonymize signups of old events by deleting name and email information. However, there may be personal data in other questions as well, for example phone number or Telegram username, which can be used to identify the person. Instead of censoring some fields, all signups could be deleted automatically instead. Storing old event information without personal data (signups) can be beneficial, as they can be used as a reference when creating new events.