Stop page extending out of viewport width by 40px

Currently the `.container` and `.wrapper-footer` elements are set to `width: 100%`, and also have horizontal padding added to them. This results in a total width of greater than the full width of the viewport; This causing the browser to scroll horizonally into empty space, regardless of how wide the viewport is.

I had added the `box-sizing: border-box` to these classes in order to resolve this.

Cheers,
Louis

P.S. I've emailed my contributor agreement to Jennifer. :paperclip: 
When we use this Box Model reset technique elsewhere in the platform, we call the Bourbon Sass ```box-sizing()``` mixin (http://bourbon.io/docs/#box-sizing) - (see [Studio layouts](https://github.com/edx/edx-platform/blob/master/cms/static/sass/elements/_layout.scss#L209) as an example.) While [browser support is fairly stable](http://caniuse.com/#feat=css3-boxsizing) for the native box-sizing property you have here, would you mind changing this to use the Bourbon mixing instead for consistency? 

We can make the universal call to not use the mixin as a browser support crutch in the future in a separate PR throughout the apps/platform.