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/plugins/sendmail/views.py

@@ -65,9 +65,11 @@ class SenderView(EventPermissionRequiredMixin, FormView):
             statusq |= Q(status=Order.STATUS_PENDING, expires__lt=now())
         orders = qs.filter(statusq)
         if form.cleaned_data.get('item'):
-            orders = orders.filter(positions__item=form.cleaned_data.get('item'))
+            orders = orders.filter(all_positions__item=form.cleaned_data.get('item'),
+                                   all_positions__canceled=False)
         if form.cleaned_data.get('subevent'):
-            orders = orders.filter(positions__subevent__in=(form.cleaned_data.get('subevent'),))
+            orders = orders.filter(all_positions__subevent__in=(form.cleaned_data.get('subevent'),),
+                                   all_positions__canceled=False)
         orders = orders.distinct()
 
         self.output = {}