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/control/templates/pretixcontrol/subevents/bulk.html

@@ -434,6 +434,12 @@
                     </button>
                 </p>
         </fieldset>
+        <fieldset>
+            <legend>{% trans "Additional settings" %}</legend>
+            {% for f in plugin_forms %}
+                {% bootstrap_form f layout="control" %}
+            {% endfor %}
+        </fieldset>
         <div class="form-group submit-group">
             <button type="submit" class="btn btn-primary btn-save">
                 {% trans "Save" %}