stage 1 of gallery reskin

![image](https://cloud.githubusercontent.com/assets/960919/3934537/8a17d768-248c-11e4-84ea-d0bfff21291f.png)

![image](https://cloud.githubusercontent.com/assets/960919/3934555/bf8b68ec-248c-11e4-96e8-2843ea26d376.png)

![image](https://cloud.githubusercontent.com/assets/960919/3934545/a3efb1a6-248c-11e4-875c-5719bd239cd4.png)

![image](http://media3.giphy.com/media/bPgNdoNa4TvB6/200w.gif)

We should probably just add an `isMedia` Boolean property to the [`Media`](https://github.com/guardian/frontend/blob/master/common/app/model/content.scala#L419) model to avoid this getting messy.