[updated] Expanding reading story packages

Due to layouts changes needed to do this work, this pull request contains many different additions. Sorry about this bulky payload.

### Global changes
- [Fix] gallery section font size (was, 18px changed back to 20px like everywhere else)
- [Refactor] Use one class only to propagate the middle column styles: `article__main-column`

### Behind a test switch
- [New] expander in trails with a thumbnail
- [New] fake content (lorem ipsum) in trails with a thumbnail
- [New] AB test for expandable trails


![screen shot 2013-08-19 at 15 45 53](https://f.cloud.github.com/assets/85783/986692/3e845a9e-08de-11e3-8ef9-91122697842e.PNG)

![screen shot 2013-08-19 at 15 46 35](https://f.cloud.github.com/assets/85783/986689/3b01ed6e-08de-11e3-956b-ab5911e8dd5c.PNG)




![hmm](http://31.media.tumblr.com/tumblr_mc2m1gvthj1rqrt0mo1_500.gif)