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/services/checkin.py

@@ -78,7 +78,7 @@ def perform_checkin(op: OrderPosition, clist: CheckinList, given_answers: dict,
     dt = datetime or now()
 
     # Fetch order position with related objects
-    op = OrderPosition.objects.select_related(
+    op = OrderPosition.all.select_related(
         'item', 'variation', 'order', 'addon_to'
     ).prefetch_related(
         'item__questions',
@@ -90,6 +90,12 @@ def perform_checkin(op: OrderPosition, clist: CheckinList, given_answers: dict,
         'answers'
     ).get(pk=op.pk)
 
+    if op.canceled:
+        raise CheckInError(
+            _('This order position has been canceled.'),
+            'unpaid'
+        )
+
     answers = {a.question: a for a in op.answers.all()}
     require_answers = []
     for q in op.item.checkin_questions: