Stop page extending out of viewport width by 40px

Currently the .container and .wrapper-footer elements are set to 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 elements in order to
resolve this.

Cheers,
Louis
Add Louis Pilfold to AUTHORS
Use box-sizing SCSS mixin rather than property
Merge pull request #5563 from lpil/master

Stop page extending out of viewport width by 40px