Temporary fix for live stream interactive embeds

This is a temporary fix for a [live-stream event tomorrow ](http://preview.gutools.co.uk/stage/ng-interactive/2014/oct/16/watch-speak-bitterness-by-forced-entertainment-live-stream), after discussing with @mattosborn we thought it would be too dangerous to apply `.u-responsive-ratio` to all `.element-embeds` as could potentially break many other interactives. 

Therefore have gone for the hacky but more resilient route of using the `data` attribute as the selector to extend.
nvm see this was copypasted from the helper class. can't put this in `_helper.scss` and access it from `_interactive.scss` ?