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/control/logdisplay.py

@@ -63,7 +63,7 @@ def _display_order_changed(event: Event, logentry: LogEntry):
         old_item = str(event.items.get(pk=data['old_item']))
         if data['old_variation']:
             old_item += ' - ' + str(ItemVariation.objects.get(pk=data['old_variation']))
-        return text + ' ' + _('Position #{posid} ({old_item}, {old_price}) removed.').format(
+        return text + ' ' + _('Position #{posid} ({old_item}, {old_price}) canceled.').format(
             posid=data.get('positionid', '?'),
             old_item=old_item,
             old_price=money_filter(Decimal(data['old_price']), event.currency),