Move to subfolders for preview files

Else the number of files can grow very large very quickly in the preview
folder. Esp on large systems.

This generates the md5 of the fileid. And then creates folders of the
first 7 charts. In that folder is then a folder with the fileid. And
inside there are the previews.

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>