Implementing new website design

Implements the new MyBB website design, referencing CSS resources from the mybb/mybb-com repository. A few other changes include modifying the name global URL variable, adding a global variable for the main site URL, and adding categories to most (if not all pages) so they can be cleanly organized on the index page (which is obviously a work in progress).

(My fork of the repository is accessible at http://jsoltesz.com/mybb.github.io)