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 2013-08-14 at 15 29 54](https://f.cloud.github.com/assets/85783/962392/d7d89c94-04ee-11e3-8b3c-d2e6924944f3.PNG)

![screen shot 2013-08-14 at 15 30 08](https://f.cloud.github.com/assets/85783/962389/d2c01d0e-04ee-11e3-8f1a-55dcd5b59196.PNG)


![hmm](http://31.media.tumblr.com/tumblr_mc2m1gvthj1rqrt0mo1_500.gif)