Audio tracking

Two audio specific tracking events - event19 for audio start, event20 for audio end

No ads on audio, and weren't not interested in quartile/duration (and possibly no interested about those things on video either, but that's another story)
only binding diagnostic events if `video` - https://github.com/guardian/frontend/blob/audio-tracking/common/app/assets/javascripts/bootstraps/media.js#L278