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...

@dmlap Sorry for the slow movement on this. Yea I'm up for giving that a go. In crunch-time trying to hit a launch deadline this week so will pick it up next week.