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 
You might want to hide this at smaller breakpoints, because on some phones the slate will load in landscape, but then what happens of the user goes back to portrait mode?