Move celery once config into own package

tests/celery.py

@@ -21,7 +21,7 @@ app.conf.task_default_priority = 5  # anything called with the task.delay() will
 app.conf.worker_prefetch_multiplier = 1  # only prefetch single tasks at a time on the workers so that prio tasks happen
 
 app.conf.ONCE = {
-    'backend': 'allianceauth.services.tasks.DjangoBackend',
+    'backend': 'allianceauth.services.celery_once.backends.DjangoBackend',
     'settings': {}
 }