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/presale/views/widget.py

@@ -515,6 +515,8 @@ class WidgetAPIProductList(EventListMixin, View):
             data['display_add_to_cart'] = False
             data['itemnum'] = 0
 
+        data['has_seating_plan'] = ev.seating_plan is not None
+
         vouchers_exist = self.request.event.get_cache().get('vouchers_exist')
         if vouchers_exist is None:
             vouchers_exist = self.request.event.vouchers.exists()