Add support for reserved seating (#1228)

* Initial work on seating

* Add seat guids

* Add product_list_top

* CartAdd: Ignore item when a seat is passed

* Cart display

* product_list_top → render_seating_plan

* Render seating plan in voucher redemption

* Fix failing tests

* Add tests for extending cart positions with seats

* Add subevent_forms to docs

* Update schema, migrations

* Dealing with expired orders

* steps to order change

* Change order positions

* Allow to add seats

* tests for ocm

* Fix things after rebase

* Seating plans API

* Add more tests for cart behaviour

* Widget support

* Adjust widget tests

* Re-enable CSP

* Update schema

* Api: position.seat

* Add guid to word list

* API: (sub)event.seating_plan

* Vali fixes

* Fix api

* Fix reference in test

* Fix test for real

src/pretix/api/views/cart.py

@@ -24,7 +24,7 @@ class CartPositionViewSet(CreateModelMixin, DestroyModelMixin, viewsets.ReadOnly
         return CartPosition.objects.filter(
             event=self.request.event,
             cart_id__endswith="@api"
-        )
+        ).select_related('seat').prefetch_related('answers')
 
     def get_serializer_context(self):
         ctx = super().get_serializer_context()