remove adblock styles when leaving ad-playback state

Adblock (and possibly other adblockers?) use `!important` element level styles to hide adverts. These need to be removed for the content to be visible, so this change cleans them up when leaving the `ad-playback` state. Not ideal but I'm not sure there is a better way of dealing with this...

I completely understand your reservations though. It would be good to find a better solution. 

It might be better to store the value of the style attribute when entering the ad-playback state and restore it when leaving.