Updates and fixes, including improved file storage

From now on files are stored in a way that should avoid various issues related
to having too many files in same location; files are still stored under assets
directory, but in similar way that Git does, i.e. picture.png would end up as
/pi/picture.png, dummy.doc as /du/dummy.doc etc.Image variations also have
their own directories now (/pi/variations/picture.0x100.png etc.)