Video end slate

This adds a end slate to our video player which displays onward journey suggestions based on your current video. It will display series content if available, else it will fallback to section content.

*Note:* for space reasons this is only loaded from tablet breakpoints upward.

- Refactor most viewed videos container to abstract the trail styles into a new `video-item` object
- JavaScript to fetch and inject into the player
- Controller actions for onward server

### Screenshots

#### Series
![google chrome](https://cloud.githubusercontent.com/assets/80089/3686728/312c7360-131c-11e4-89a5-b26633a12f7d.png)

#### Section
![google chrome](https://cloud.githubusercontent.com/assets/80089/3686727/2e99fe2e-131c-11e4-8caa-0ef0784533d8.png)

/cc @rich-nguyen @mattosborn 
With the [new font-scale](https://raw.githubusercontent.com/guardian/guss-typography/master/font-scale.png),

18px = `get-line-height($fs-headline, 1)`