Use Django's cache framework for task keys.

Remove depreciated only_one decorator.

Prevent including task_self repr in key name.

Because some tasks are nested in a class, they use a task_self argument instead of the normal self which the celery_once package doesn't recognize to strip out.

tests/celery.py

@@ -1,4 +1,4 @@
-import os, sys
+import os
 from celery import Celery
 
 # set the default Django settings module for the 'celery' program.
@@ -7,17 +7,15 @@ os.environ.setdefault('DJANGO_SETTINGS_MODULE', 'settings')
 from django.conf import settings  # noqa
 
 app = Celery('devauth')
-app.conf.ONCE = {
-  'backend': 'celery_once.backends.Redis',
-  'settings': {
-    'url': 'redis://localhost:6379/0',
-    'default_timeout': 60 * 60
-  }
-}
 
 # Using a string here means the worker don't have to serialize
 # the configuration object to child processes.
 app.config_from_object('django.conf:settings')
+app.conf.ONCE = {
+    'backend': 'allianceauth.services.tasks.DjangoBackend',
+    'settings': {}
+}
 
 # Load task modules from all registered Django app configs.
 app.autodiscover_tasks(lambda: settings.INSTALLED_APPS)
+