Lightbox for article images

Article images now have an icon to launch all of the article's images in a lightbox view:

![screen shot 2014-11-18 at 11 22 30](https://cloud.githubusercontent.com/assets/2236852/5104784/5a49b8f2-6fdd-11e4-845d-da916f88f997.png)
![screen shot 2014-11-18 at 11 22 39](https://cloud.githubusercontent.com/assets/2236852/5104785/5b4ac7a0-6fdd-11e4-8780-be4087c71a10.png)

cc @mattosborn 
is this going to run on all pages now? can't think where else we use the index query parameter but might want to double check there aren't any side effects. don't want to push history state without launching the lightbox.