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/checkin.py

@@ -231,7 +231,7 @@ class CheckinListPositionViewSet(viewsets.ReadOnlyModelViewSet):
                     )
                 ))
             ).select_related(
-                'item', 'variation', 'item__category', 'addon_to', 'order', 'order__invoice_address'
+                'item', 'variation', 'item__category', 'addon_to', 'order', 'order__invoice_address', 'seat'
             )
         else:
             qs = qs.prefetch_related(
@@ -241,7 +241,7 @@ class CheckinListPositionViewSet(viewsets.ReadOnlyModelViewSet):
                 ),
                 'answers', 'answers__options', 'answers__question',
                 Prefetch('addons', OrderPosition.objects.select_related('item', 'variation'))
-            ).select_related('item', 'variation', 'order', 'addon_to', 'order__invoice_address', 'order')
+            ).select_related('item', 'variation', 'order', 'addon_to', 'order__invoice_address', 'order', 'seat')
 
         if not self.checkinlist.all_products:
             qs = qs.filter(item__in=self.checkinlist.limit_products.values_list('id', flat=True))