IMPROVEMENT: Add management command to abort all active jobs

[kimakan]

Before a downtime, it is necessary to abort the active jobs to ensure a defined job status after the maintenance.

python manage.py abort_active_jobs

The jobs should get the status ABORTED with the error message Aborted due to a planned downtime. Alternatively, the message could be provided with an option

python manage.py abort_active_jobs -m "Aborted due to the maintenance"