Fixes website issues caused by parallax component.

First this fixes #161 by checking for the existence of an element with the id `js-parallax-window` before allowing parallax() to be called, preventing `Uncaught TypeError: Cannot read property 'top' of undefined` errors on the website.

It also has the added benefit of fixing the slide out menus on the patterns and type systems pages of the website, which is what I really wanted working.