Allow customers to choose to receive their refund as a gift card (#1626)

* Minor text adjustments

* Allow users to receive their cancellation as a gift card

src/pretix/api/serializers/event.py

@@ -612,6 +612,7 @@ class EventSettingsSerializer(serializers.Serializer):
         'cancel_allow_user_paid_keep_fees',
         'cancel_allow_user_paid_keep_percentage',
         'cancel_allow_user_paid_adjust_fees',
+        'cancel_allow_user_paid_refund_as_giftcard',
     ]
 
     def __init__(self, *args, **kwargs):