Right hand most popular component

This introduces an A/B test which displays a most popular component in the right hand column of articles at desktop breakpoints. The component has two separate design variants: a list view and a trail image view. Each design variant will also be tested with varying amounts of items: 3, 5, 6, 10

Full details of the test an desired outcomes can be found [here](https://docs.google.com/a/guardian.co.uk/spreadsheet/ccc?key=0ApW5rEdUdSnadGFzWEdfWE1jTnRwZUtjb21DcVVtenc)

/cc @rich-nguyen 

### List view
![screenshot from 2013-12-17 15 11 30](https://f.cloud.github.com/assets/80089/1765005/8485b1c8-672f-11e3-820c-cb76e0f3f6d4.png)


### Image view
![screenshot from 2013-12-17 15 03 21](https://f.cloud.github.com/assets/80089/1765007/8a860348-672f-11e3-8eb1-645722678be3.png)