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: 
Similar to  [my previous note](https://github.com/edx/edx-platform/pull/5563/files#r19000048) above.