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/static/pretixbase/scss/_theme.scss

@@ -1,6 +1,13 @@
 .sidebar-nav li > a > .fa {
   color: $navbar-inverse-bg;
 }
+.sidebar-nav li > a > svg {
+  position: relative;
+  top: 3px;
+  path {
+    fill: $navbar-inverse-bg;
+  }
+}
 .nav .testmode a {
   background: $brand-warning;
   color: black;
@@ -168,6 +175,11 @@
   border: 1px solid #ddd;
 }
 
+svg.svg-icon {
+  position: relative;
+  top: 2px;
+}
+
 @include table-row-variant('success', lighten($brand-success, 40%));
 @include table-row-variant('info', lighten($brand-info, 30%));
 @include table-row-variant('warning', lighten($brand-warning, 40%));