Add manage.py commands to find and delete disconnected Reply records

[heathdutton]

Fixes #7203

find_disconnected_db_submissions() only checks for Submission records missing from the filesystem, not Reply records. This adds parallel functions to find and delete disconnected Reply records.

New commands:

• manage.py check-disconnected-db-replies
• manage.py list-disconnected-db-replies
• manage.py delete-disconnected-db-replies [--force]

Test plan

1. Register as a source and submit something
2. Reply to the source
3. Delete that source's directory from /var/lib/securedrop/store
4. Run manage.py check-disconnected-db-replies - should report disconnected replies
5. Run manage.py list-disconnected-db-replies - should list the reply ID
6. Run manage.py delete-disconnected-db-replies --force - should delete the reply record

Checklist

This change accounts for:

• any required additional documentation
• any necessary AppArmor changes (added or removed application files)
• any impact on new SecureDrop installs and upgrades
• our dependency update policy

[zenmonkeykstop]

Hi @heathdutton - thanks for this one, review in progress. These cleanup commands are intended to be mostly used via a systemd timed job - if you're interested in how that works take a look at the service definitions under https://github.com/freedomofpress/securedrop/tree/develop/securedrop/debian/app-code/lib/systemd/system . If not, no worries, we can add that part in a subsequent PR.