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/templates/pretixpresale/event/voucher.html

@@ -29,6 +29,14 @@
             <input type="hidden" name="subevent" value="{{ subevent.id|default_if_none:"" }}" />
             <input type="hidden" name="_voucher_code" value="{{ voucher.code }}">
 
+            {% if voucher.seating_available %}
+                {% if event.has_subevents %}
+                    {% eventsignal event "pretix.presale.signals.render_seating_plan" request=request subevent=subevent voucher=voucher %}
+                {% else %}
+                    {% eventsignal event "pretix.presale.signals.render_seating_plan" request=request voucher=voucher %}
+                {% endif %}
+            {% endif %}
+
             {% for tup in items_by_category %}
                 <section>
                     {% if tup.0 %}