Refactor cancelling positions and orders in the data model (#1088)

- [x] Data model
- [x] display in order view in backend
- [x] review all usages of OrderPositions.objects
- [x] review all usages of order.positions
- [x] review all other model usages
- [x] review plugins
- [x] plugins backwards-compatible API?
- [x] decide on way forward for REST API
- [x] need to cancel fees
- [x] tests
- [ ] plugins
  - [ ] gdpr
  - [ ] reports
- [x] docs

src/pretix/base/models/event.py

@@ -355,7 +355,8 @@ class Event(EventMixin, LoggedModel):
         if not really:
             raise TypeError("Pass really=True as a parameter.")
 
-        OrderPosition.objects.filter(order__event=self).delete()
+        OrderPosition.all.filter(order__event=self, addon_to__isnull=False).delete()
+        OrderPosition.all.filter(order__event=self).delete()
         OrderFee.objects.filter(order__event=self).delete()
         OrderPayment.objects.filter(order__event=self).delete()
         OrderRefund.objects.filter(order__event=self).delete()