Implement sharing to unregistered users

[rrrnld]

[rrrnld]

Thanks to https://github.com/airsonic/airsonic/pull/962 there are now links to the songs in the markup. We can probably fetch the html and query for the source elements, which is hacky but gives us an option to implement this until there's a proper API.

[almet]

That would be so cool :-)