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_orders.py

@@ -616,6 +616,26 @@ class OrderChangeManagerTests(TestCase):
         self.order.refresh_from_db()
         assert self.order.positions.count() == 1
         assert self.order.total == self.op2.price
+        self.op1.refresh_from_db()
+        assert self.op1.canceled
+
+    def test_cancel_with_addon(self):
+        self.shirt.category = self.event.categories.create(name='Add-ons', is_addon=True)
+        self.ticket.addons.create(addon_category=self.shirt.category)
+        self.ocm.add_position(self.shirt, None, Decimal('13.00'), self.op1)
+        self.ocm.commit()
+        self.order.refresh_from_db()
+        self.ocm = OrderChangeManager(self.order, None)
+        assert self.order.positions.count() == 3
+
+        self.ocm.cancel(self.op1)
+        self.ocm.commit()
+        self.order.refresh_from_db()
+        assert self.order.positions.count() == 1
+        assert self.order.total == self.op2.price
+        self.op1.refresh_from_db()
+        assert self.op1.canceled
+        assert self.op1.addons.first().canceled
 
     def test_free_to_paid(self):
         self.order.status = Order.STATUS_PAID