finalize asset storage strategy

[schlawg]

existing (dummy) bot assets are temporarily housed in lifat and available to clients as https://lichess1.org/assets/lifat/bots//

there is tentative agreement to use a dedicated filesystem location residing or symlinked as public/user. "user" was chosen arbitrarily, i am not attached - something like "user-data" or just "data" is also fine

this will impact urls, bin/deploy, etc. we seem stuck with lifat for now so near term bin/deploy could just rsync ../lifat/bots to public/user whenever it runs.

filenames are hashes so assets in this folder will never be overwritten. directories can change in the future.