Self-service refund form (#1135)

* Auto-refund

* Add missing template

* Notification for requested refund

* Model-level tests

* Add front-end tests

* Default to notify

src/pretix/base/notifications.py

@@ -241,6 +241,12 @@ def register_default_notification_types(sender, **kwargs):
             _('External refund of payment'),
             _('An external refund for {order.code} has occurred.')
         ),
+        ParametrizedOrderNotificationType(
+            sender,
+            'pretix.event.order.refund.requested',
+            _('Refund requested'),
+            _('You have been requested to issue a refund for {order.code}.')
+        ),
         ActionRequiredNotificationType(
             sender,
         )