Add Download links for album, playlist

[muellerj]

Based on the work already done in #350.

[muellerj]

TODO: Add the same functionality to playlists.

I wanted to implement this as well, but the songsManager currently seems to have no information about the membership of songs?

[muellerj]

Update: I wrote a second patch which passes the required information along with each song and thus allows downloads of playlists as well. The /sync-endpoint thereby becomes slightly heavier, but overall the change is small.

Please review and let me know what you think.

[muellerj]

The settings seem to work consistently on Desktop (Chrome, Firefox, Safari) but I’ve not had reproducible results on iOS (some downloads seem to get skipped). Help is appreciated.

The obvious conceptual alternative is to create a zip file server-side and download that, but that’s a lot more involved in terms of code changes.

[amitjethani]

Would love to see this get merged in.