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/settings.py

@@ -1,6 +1,7 @@
 import json
 from collections import OrderedDict
 from datetime import datetime
+from decimal import Decimal
 from typing import Any
 
 from django.conf import settings
@@ -224,6 +225,30 @@ DEFAULTS = {
         'default': 'True',
         'type': bool
     },
+    'cancel_allow_user_until': {
+        'default': None,
+        'type': RelativeDateWrapper,
+    },
+    'cancel_allow_user_paid': {
+        'default': 'False',
+        'type': bool,
+    },
+    'cancel_allow_user_paid_keep': {
+        'default': '0.00',
+        'type': Decimal,
+    },
+    'cancel_allow_user_paid_keep_fees': {
+        'default': 'False',
+        'type': bool,
+    },
+    'cancel_allow_user_paid_keep_percentage': {
+        'default': '0.00',
+        'type': Decimal,
+    },
+    'cancel_allow_user_paid_until': {
+        'default': None,
+        'type': RelativeDateWrapper,
+    },
     'contact_mail': {
         'default': None,
         'type': str