Allow Individual Tracks as Custom Seeds

[featurebase-for-github]

I want to be able to add individual songs/tracks as custom seeds, not just artists. This feature was working perfectly before, but now it gives a "Server error (500)" when I try to search for a track to add. It's frustrating because this functionality was available from the beginning and now it's gone. This is a critical feature for creating playlists with the right vibe.