Add functionality to download music

[agersant]

• Individual song
• Whole directory
• Whole album
• Whole playlist

Implementation notes:

• Probably would best be added in right-click context menus in web UI
• Look into streaming zip responses without making a temp file on server

[lnicola]

Look into streaming zip responses without making a temp file on server

I thought this wasn't possible because the local file headers include the uncompressed length. But for music it's probably fine to used "stored" (no-op) compression, and ZIP64 might actually put this in an extra descriptor which sits after the file data.

Anyway, for a similar application I stream TAR, which doesn't have this issue.