Linked content cards in sidebar

# Cards to linked content

This PR is transforming the links to other articles in the body content into cards to the left sidebar for desktop users (every 4 paragraphs or so).

## Hypothesis

People who are presented with inline links to Guardian content
as cards in the left sidebar will read more linked content.

## Buckets

- Variant A: Viewing the original (50%)
- Variant B: Viewing linked content as cards in sidebar (50%)

## Predictions

### Matt C

- Number of read linked content increases by 3%

### Patrick

- Number of read linked content increases by 15%

### Chris M

- Number of read linked content increases by 30%

### Kaelig

- Number of read linked content increases by 200%


![screen shot 2013-08-06 at 17 15 11](https://f.cloud.github.com/assets/85783/918369/6acb6a14-feb3-11e2-9392-6c2cf9a7913a.PNG)


![Thanks, David](http://i.imgur.com/MUgdLh7.gif)