image-cache: use sha256 hash of url for key

[v0l]

closes #630

Also added sub-dirs to avoid very large file numbers in a single dir which can cause issues down the line.

[jb55]

Looks good, the only thing stopping me from merging this outright is lack of migration step: Algo: 1. If it detects the previous path, move it to the new location This will allow us to not redownload things and gracefully migrate existing installs

[v0l]

Ok maybe we should also consider tracking image cache usage so that we could apply some limits

[jb55]

ok we'll just do the migration for awhile and then eventually remove it