Figure out how to handle character avatars and implement

Images are binary blobs so the obvious go-to is S3. A couple of things to consider though:

- I want self-hosting to be an option for the entire service, so AWS lock-in is not acceptable. `minio` can help us out here.
- For actually running the official service, we'll likely want to make use of Cloudflare's free CDN to cut down on bandwidth and load.