Optimize ES6 Module Loader Polyfill

Optimize ES6 Module Loader Polyfill

This commit makes the ES6 module loader polyfill use Web Workers,
so that Babel doesn't block the browser from animating.  It also
uses localStorage to cache the compiled results, only recompiling
on source changes, so it makes loading faster while developing noVNC.

This includes a vendored copy of the ES6 module loader, modified as
described above.