Adding a song as guest should consume a vote

[flagbug]

Guests should be able to add a song to the playlist, be it from the computer or from their phone, but doing so should consume a "shadow vote".

A "shadow vote" shouldn't add a vote directly on the song, but it should decrease the user's vote count. The shadow vote should be restored after the song is played,just like a normal vote behaves.

[flagbug]

Fixed with #110