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.
Ahh yes I just copy and pasted this from [here](https://github.com/guardian/frontend/blob/master/static/src/stylesheets/base/_helpers.scss#L98), where it has a comment block above it. Didn't really want to remove that code as it makes sense living in the `helpers.scss`.