Include main image in article lightbox

This pull request includes some refactoring of the article lightbox so that the main image is now included if it is of sufficient size. 

It also adds a new adjective to our codebase: "lightboxable", which describes an image that is over 620px wide. 

![screen shot 2014-12-04 at 11 25 34](https://cloud.githubusercontent.com/assets/2236852/5297591/5b1690bc-7ba8-11e4-867a-cedbc3d26abb.png)

Also a couple of changes to the width of the figcaptions, as requested by Chris, along with a fix for some icon overlapping.