Submissions can be posted long after the staging files have been purged

[jennifer-richards]

Describe the issue

In #7454, a submission was approved 60 days after it was submitted. Files older than 14 days are regularly purged from the staging directory where they are stored pending approval. The result was that the approved draft was posted but, because the staged files had long before been deleted, the actual contents are missing.

Stale submissions should be canceled on a schedule that is coordinated with the staging file purge. A submission should be canceled before its files are removed.

We already have a periodic Celery task that cancels submissions that have not yet been validated - a similar task should cancel stale submissions. It'd be adequate to run the new task on the daily schedule.

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