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_invoices.py

@@ -58,6 +58,14 @@ def env():
         price=Decimal("42.00"),
         positionid=2,
     )
+    OrderPosition.objects.create(
+        order=o,
+        item=t_shirt,
+        variation=variation,
+        price=Decimal("42.00"),
+        positionid=3,
+        canceled=True
+    )
     gs = GlobalSettingsObject()
     gs.settings.ecb_rates_date = date.today()
     gs.settings.ecb_rates_dict = json.dumps({