Move globals to one config object

We've had a few globals floating around, and even an include with one element and an data-attribute.  Messy.  This creates a config "window.mdn" object within the base template.  This object will have a few properties to start and which will have properties added at different times.

Eventually this will be moved to an external JS file in AMD format, but until we make that switch, base is the best place to keep this.  We could move it to a config.js now, but that's another request and we'd want it preprocessed, making it slow the page down more.