Support backends other than S3

[MoralCode]

The next issue is that most people don't have access to an S3 instance but that's something you don't need access to the VPN for, and we can give out credentials for our development S3 bucket.

Originally posted by @wilsonmcdade in #9 (comment)

I recently had the idea to reuse this codebase for another project, and would like to avoid using S3 if i can. While thats partly my fault (i should prob look for a FOSS, S3-compatible storage layer, ideally one that runs as a container) i think it would be cool to also allow things like the filesystem to be used for storing simple images. If theres a need for this, I might end up working on that