allow for events to be cancelled

[jeriox]

closes#373

[jeriox]

@felixrindt I'm not sure how we want to proceed with a delete button for the event. Should we always offer both options? Or only allow the event to be deleted without being cancelled first when there are no participations?

[coveralls]

Coverage: 87.697% (-0.3%) from 87.95% when pulling dd99b80 on event-cancellation into 8389f0f on main.

[felixrindt]

@felixrindt I'm not sure how we want to proceed with a delete button for the event. Should we always offer both options? Or only allow the event to be deleted without being cancelled first when there are no participations?

I think we should allow both, that is:

• deleting an event entirely, even if it has participations without sending an email
• the more graceful approach introduced in this PR.

Maybe we can join the two together into one form. Also, what about the bulk delete action? Should it work on events with participations?

[jeriox]

@felixrindt I'm not sure how we want to proceed with a delete button for the event. Should we always offer both options? Or only allow the event to be deleted without being cancelled first when there are no participations?

I think we should allow both, that is:

* deleting an event entirely, even if it has participations without sending an email
* the more graceful approach introduced in this PR.

Maybe we can join the two together into one form. Also, what about the bulk delete action? Should it work on events with participations?

I like that idea. The user could then only have one button with "delete event" and can check a checkbox in the delete view with "notify users about the cancellation". if checked, the textarea for the message appears.

I don't see a reason why the bulk action shouldn't work?