Configuration cleanup (aka stop hardcoding /data)

Put the configuration for shared blobs, shared uploads, and the SQLite DB all into app.config where they belong. Can be overridden via environment variable at runtime. Move the mkdir calls into a flask command (init-dirs) rather than unconditionally running them. In the Docker container, we already create them for you anyway. While we're here, genenralize the DB config to just be a connection string instead of a specific path on disk. This enables non-SQLite implementations, or using SQLite :memory: DBs for eg Alembic down the road or something.