Fix wrong question when track media is not available

[4www]

Sometimes the quiz is on a track which media cannot be played by the radio4000-player.

We should listen on the mediaNotAvailable event sent by the player, and refresh the question.

[oskarrough]

Yes. And initially filter out unavailable tracks in https://github.com/internet4000/trivia-radio4000/blob/master/app/routes/quiz/questions.js#L7