SUPPLEMENTALLY:
- Add an `expiration` field to the `file` table of the database
- Produce a migration for the above change
- Overhaul the cleanup script, and integrate into fhost.py
(now run using FLASK_APP=fhost flask prune)
- Replace the old cleanup script with a deprecation notice
- Add information about how to expire files to the index
- Update the README with information about the new script