External link styling

Mostly from messrs @robertberry and @sndrs 

![screen shot 2014-11-24 at 18 36 21](https://cloud.githubusercontent.com/assets/1147913/5170588/c8ebbd44-7408-11e4-9d10-a5fb8cb3b2f4.png)
We already maintain a list of trusted internal origins [here](https://github.com/guardian/frontend/blob/master/common/conf/env/PROD.properties#L24). Would be good to not duplicate.