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...

@mattosborn I like the idea of storing the tech styles as part of the snapshot. Would you be up for trying that out?