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

doc/api/resources/checkinlists.rst

@@ -396,6 +396,7 @@ Order position endpoints
             "addon_to": null,
             "subevent": null,
             "pseudonymization_id": "MQLJvANO3B",
+            "seat": null,
             "checkins": [
               {
                 "list": 1,
@@ -505,6 +506,7 @@ Order position endpoints
         "addon_to": null,
         "subevent": null,
         "pseudonymization_id": "MQLJvANO3B",
+        "seat": null,
         "checkins": [
           {
             "list": 1,