A Pending Scan Task Freezes the Queue if its Associated Scan is Deleted

[Matthew-Grayson]

🐛 Summary

If a scan task is pending when its associated scan is deleted, the queuing system will freeze and prevent other scans from running.

To reproduce

As a Global User, navigate to the scans pages and create a scan.
Click the run button several times and delete the scan.
Create a new scan and click the run button.
Confirm that the new scan doesn't run.
You may also look at the scan_task table to confirm that the old scan tasks have a pending status.

Expected behavior

I suggest we add logic to the scheduler to mark the orphaned scan tasks as finished and/or delete them.