Event can be changed from private signups to public, which reveals user data

[ilarikuoppala]

Currently it is possible that a user signs up to an event assuming the event signups are private, but the event can then be changed to have public signups. This reveals user data without asking permission from the user.

Could be fixed either by preventing changing private events to public, or storing the publicity setting of each signup separately and asking that from the users beforehand.