Put files on the filesystem instead of in the database

[praseodym]

Currently CHoice saves the exams in its PostgreSQL database. That database is currently ~700MB in size, which causes a lot of churn backups because files cannot be effectively deduplicated. It also just isn't a great idea to store large files in a database anyway.

I'd suggest the exams be moved to the filesystem (a Kubernetes persistent volume) or e.g. a Google Cloud Storage bucket whenever we're migrated into the cloud.