Add system question order to device event settings api endpoint (#3186)

src/pretix/api/serializers/event.py

@@ -863,6 +863,7 @@ class DeviceEventSettingsSerializer(EventSettingsSerializer):
         'invoice_address_from_tax_id',
         'invoice_address_from_vat_id',
         'name_scheme',
+        'system_question_order',
     ]
 
     def __init__(self, *args, **kwargs):

src/pretix/base/settings.py

@@ -212,6 +212,8 @@ DEFAULTS = {
     'system_question_order': {
         'default': {},
         'type': dict,
+        'serializer_class': serializers.DictField,
+        'serializer_kwargs': lambda: dict(read_only=True, allow_empty=True),
     },
     'attendee_names_asked': {
         'default': 'True',