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/event.py

@@ -86,7 +86,7 @@ class EventViewSet(viewsets.ModelViewSet):
             )
 
         return qs.prefetch_related(
-            'meta_values', 'meta_values__property'
+            'meta_values', 'meta_values__property', 'seat_category_mappings'
         )
 
     def perform_update(self, serializer):
@@ -242,7 +242,7 @@ class SubEventViewSet(ConditionalListView, viewsets.ModelViewSet):
                 event__in=self.request.user.get_events_with_any_permission()
             )
         return qs.prefetch_related(
-            'subeventitem_set', 'subeventitemvariation_set'
+            'subeventitem_set', 'subeventitemvariation_set', 'seat_category_mappings'
         )
 
     def perform_update(self, serializer):