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/tests/base/test_models.py

@@ -740,7 +740,7 @@ class OrderTestCase(BaseQuotaTestCase):
         q = Question.objects.create(question='Foo', type=Question.TYPE_BOOLEAN, event=self.event)
         self.item1.questions.add(q)
         assert self.order.can_modify_answers
-        self.order.status = Order.STATUS_REFUNDED
+        self.order.status = Order.STATUS_CANCELED
         assert not self.order.can_modify_answers
         self.order.status = Order.STATUS_PAID
         assert self.order.can_modify_answers
@@ -963,7 +963,7 @@ class OrderTestCase(BaseQuotaTestCase):
         assert o.has_external_refund
 
     def test_pending_order_pending_refund(self):
-        self.order.status = Order.STATUS_REFUNDED
+        self.order.status = Order.STATUS_CANCELED
         self.order.save()
         self.order.payments.create(
             amount=Decimal('46.00'),
@@ -1023,6 +1023,14 @@ class OrderTestCase(BaseQuotaTestCase):
         assert not o.has_pending_refund
         assert not o.has_external_refund
 
+    def test_canceled_positions(self):
+        self.op1.canceled = True
+        self.op1.save()
+        assert OrderPosition.objects.count() == 1
+        assert OrderPosition.all.count() == 2
+        assert self.order.positions.count() == 1
+        assert self.order.all_positions.count() == 2
+
 
 class ItemCategoryTest(TestCase):
     """