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/fragment_modals.html

@@ -0,0 +1,15 @@
+{% load i18n %}
+<div id="ajaxerr">
+</div>
+<div id="loadingmodal">
+    <div class="modal-card">
+        <div class="modal-card-icon">
+            <i class="fa fa-cog big-rotating-icon"></i>
+        </div>
+        <div class="modal-card-content">
+            <h3></h3>
+            <p class="text"></p>
+            <p class="status">{% trans "If this takes longer than a few minutes, please contact us." %}</p>
+        </div>
+    </div>
+</div>