CSS loading fix

`Gruntfile.js` was processing the LESS files in according to the order in `settings.json` but then concatenating the CSS files alphabetically from the filesystem. So LESS/CSS was being written to compensate for the alphabetical order.

This fixes that.

This fix does, however, come with one caveat. It removes loading `*.css` files from the assets--meaning one can only use `*.less` files when creating an add-on. However, there are no known uses of `*.css` files in an add-on, and if there are, they can be renamed to `.less` and have the exact same result--as LESS files can contain just pure CSS and have the same output (thankfully!).

@deathbearbrown @garrensmith this one's for you. :smile_cat: