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/api/serializers/order.py

@@ -324,7 +324,7 @@ class OrderPositionCreateSerializer(I18nAwareModelSerializer):
                   'secret', 'addon_to', 'subevent', 'answers')
 
     def validate_secret(self, secret):
-        if secret and OrderPosition.objects.filter(order__event=self.context['event'], secret=secret).exists():
+        if secret and OrderPosition.all.filter(order__event=self.context['event'], secret=secret).exists():
             raise ValidationError(
                 'You cannot assign a position secret that already exists.'
             )