Fix task register (#3805)

* fix schedule registration

* add collection step for tasks

* make tasks register configurable

* extend docs

* Also run InvenTree setup in testing

* fix import loading method

* fix wrong task registration

* do not test

* do only distinct testing

* ignore import error for coverage

InvenTree/common/tasks.py

@@ -5,9 +5,12 @@ from datetime import datetime, timedelta
 
 from django.core.exceptions import AppRegistryNotReady
 
+from InvenTree.tasks import ScheduledTask, scheduled_task
+
 logger = logging.getLogger('inventree')
 
 
+@scheduled_task(ScheduledTask.DAILY)
 def delete_old_notifications():
     """Remove old notifications from the database.