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/base/pdf.py

@@ -238,6 +238,11 @@ DEFAULT_VARIABLES = OrderedDict((
             "TIME_FORMAT"
         ) if ev.date_admission else ""
     }),
+    ("seat", {
+        "label": _("Seat name"),
+        "editor_sample": _("3, 4-5"),
+        "evaluate": lambda op, order, ev: str(op.seat if op.seat else _('General admission'))
+    }),
 ))