Cancel a pending/booked appointment

[MelissaAutumn]

You should be able to cancel a pending or booked appointment. This includes any appointments scheduled via general availability.

This should cascade into removing the event from the users external calendar, and cancelling any zoom meetings attached to the appointment.

This should also send an email to the attendees with a optional reason why the event was canceled.

[devmount]

This should be also available from the booking overview in #282

[devmount]

For reference: Rejecting a pending booking request, event removal and email notification for attendees is now possible with #768.

@MelissaAutumn I'm wondering about the difference of cancelling and rejecting a pending event. Despite giving an optional reason (which we could implement for rejecting a booking request too), those are somehow the same, or aren't they?

In any case, cancelling a confirmed booking still needs to be done.

[MelissaAutumn]

Yep I would say those are equivalent.