Add a workflow for appointments deleted.

Use case:

Currently if a user deletes an appointment, any workflow that the customer was in related triggered by the appointment will stop for that customer.

We have a circumstance when we need to trigger a workflow when an appointment is deleted, as well as when it is cancelled. The user needs to delete an appointment (to de-junk up their calendar), instead of just cancelling it which leaves it in the calendar.

We can trigger a workflow if the used cancels an appointment, but if the user deletes the appointment before cancelling it, we cannot currently trigger the workflow. Hence the need for this trigger.