Make manager download new art files for artist that already has art
Created by: eratosthene
Right now when the manager examines a fanart folder for an artist, if there are any files in the 'artistbackground' folder, it will not attempt to download artistbackground files for that artist. The same is true about the 'logo' and 'badge' folders for hdmusiclogo and musiclogo files. This is so we don't hammer fanart.tv and try to redownload a bunch of files that we already have. There needs to be some sort of hashing taking place so that existing files get skipped but new files will be downloaded. Files are currently downloaded to the same filename as fanart.tv uses, but are trimmed, so the filesize and checksum differ.