Fix #1378 -- API: Allow to access and modify (some) event setti… (#1569)

* API: Allow to access event settings

* Convert most "general" settings

* Smaller fixes

* Add more settings

* Relative dates, nulling

* Fix a test failure

* Fix wrong attribute access

src/tests/api/test_permissions.py

@@ -23,6 +23,8 @@ event_urls = [
 ]
 
 event_permission_sub_urls = [
+    ('get', 'can_change_event_settings', 'settings/', 200),
+    ('patch', 'can_change_event_settings', 'settings/', 200),
     ('get', 'can_view_orders', 'orders/', 200),
     ('get', 'can_view_orders', 'orderpositions/', 200),
     ('delete', 'can_change_orders', 'orderpositions/1/', 404),