Article truncation A/B test

This introduces an A/B to test the effectiveness of truncating the article at certain word counts to increase CTR on onward journeys.

Variants:
- Control
- 300 words
- 400 words
- 500 words

This also temporally moves the story-package above the comment section, but *only* for users in the test bucket. 

### Truncated
![desktop](https://f.cloud.github.com/assets/80089/2155297/23d6581e-9443-11e3-851b-7720349f7c5b.png)


/cc @rich-nguyen @mattosborn