Center lightbox with css instead of javascript and make it kinda responsive

this is a quick fix for this bug :point_right: https://trello.com/c/Ffg128Wa/317-article-layers-on-small-screens-aren-t-very-usable. The layers should have a responsive layout, but this "fix" the bug for now. 

![lightbox](https://cloud.githubusercontent.com/assets/1152640/3410237/52aee65a-fde4-11e3-8068-8546a2adbd5d.gif)