This simply updates js/utils/urls.es6.js to use export rather than

setting content in the Djblets namespace, and re-exports that in

js/utils/index.ts.
This change was used as part of the tests for ES6 modules and Rollup.js,

and can serve as a simple example for new code going forward.

Tested pages using functions here, and didn't see any problems. Verified

that the generated JavaScript for the bundle was setting the functions in

Djblets as before (though through the bundle IIFE).