Apology for invitation

[feliciaan]

Users should be able to apologise for invitations.
Sometimes users are invited via snail mail but are not able to attend the event.
But this should be saved to the backend.

Proposal: an unable to attend event ticket, this ticket has a boolean that says that the user is not able to make it to the event.

TODOs

• create ticket or other method to save it
• send confirmation mail to user that they have apologised

[TomNaessens]

Need to think about what happens when a ticket has been (partially) paid, or overpaid. We certainly need to keep track of the ticket in some way to see a history.

As a first step, we could also disable cancelling a registration for paid tickets.