Allow cancelling registrations by users

[matthijskooijman]

Ideally, users should be able to cancel their own registrations when:

• they are only prepared, not registered yet, or
• they are on the waitinglist

The former is fairly easy, the latter is a bit more involved (since it might involve transferring money back, so that would at least require notification).

Cancelling a registration that is REGISTERED probably always needs to be done by the organisers (so the player asks for this by e-mail), since additional steps must usually be taken (e.g. re-casting a character) or additional limitations might apply.

This relates to #67 and probably would be implemented after that.