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/models/items.py

@@ -442,7 +442,7 @@ class Item(LoggedModel):
     def allow_delete(self):
         from pretix.base.models.orders import OrderPosition
 
-        return not OrderPosition.objects.filter(item=self).exists()
+        return not OrderPosition.all.filter(item=self).exists()
 
     @cached_property
     def has_variations(self):