Allow to import orders (#1516)

* Allow to import orders

* seats, subevents

* Plugin support

* Add docs

* Warn about lack of quota handling

* Control interface test

* Test skeleton

* First tests for the impotr columns

* Add tests for all columns

* Fix question validation

src/pretix/base/settings.py

@@ -683,6 +683,10 @@ Your {event} team"""))
         )),
         'type': LazyI18nString
     },
+    'order_import_settings': {
+        'default': '{}',
+        'type': dict
+    },
     'organizer_info_text': {
         'default': '',
         'type': LazyI18nString