Add whatwg-fetch polyfill for older browsers like IE11

[lmorchard]

Not super happy with this, since it looks like the vendor bundle is starting to grow again. But, it fixes an error in IE11 (and I assume other older browsers). Issue #2943

[lmorchard]

But, looking at the analyzer, this polyfill only adds 12.38kb so 🤷‍♂️

[fzzzy]

At some point soon, we could start using script type="module" for browsers that support it, and script nomodule for browsers that don't, meaning we could only ship polyfills and transpiled code to older browsers. It looks like Firefox is the last browser to enable it by default, although it is available behind a flag.

http://caniuse.com/#search=script%20type%3D%22module%22