Better forum layout according to browser size

Ref: https://github.com/OpenUserJs/OpenUserJS.org/issues/435

I agree with @TimidScript that the forum could utilize it's space better on big screens.

What I did was make the menu slide to the left of the forum posts on desktop (and bigger) browsers.

![Better forum layout according to browser size](https://cloud.githubusercontent.com/assets/55841/5158823/9f3c695c-734b-11e4-8692-a95ffe9bfa45.jpg)