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/api/views/order.py

@@ -770,7 +770,7 @@ class InvoiceViewSet(viewsets.ReadOnlyModelViewSet):
             raise PermissionDenied('The invoice file is no longer stored on the server.')
         else:
             c = generate_cancellation(inv)
-            if inv.order.status not in (Order.STATUS_CANCELED, Order.STATUS_REFUNDED):
+            if inv.order.status != Order.STATUS_CANCELED:
                 inv = generate_invoice(inv.order)
             else:
                 inv = c