Alasdair/rwd trackselection

@dianakhuang @rlucioni @frrrances Updates to make track selection page responsive
I think this mixin is messing with the body width. I'm seeing the header and footer getting cut-off (see screenshots). Can we just remove it and use our own styles here?
![screen shot 2014-11-10 at 1 37 43 pm](https://cloud.githubusercontent.com/assets/4327102/4981415/cab9132a-6908-11e4-890d-984d8c03f33d.png)
![screen shot 2014-11-10 at 1 37 51 pm](https://cloud.githubusercontent.com/assets/4327102/4981414/cab60432-6908-11e4-9f0d-6ebe310e18f7.png)