Disable articles AngularJS app for non-users

The articles AngularJS app will only be active for logged-in users. The
benefits for regular users when viewing the index page and articles are
very small. The Angular app manages to adapt to browsers that can't do
HTML5 URLs, but it's slower and clunkier than regular full page loads on
Android. Even in IE, where it's not so bad, it still breaks the URLs and
disales links within pages.