This lets us remove a polyfill and some other hacks, and brings along some
additional jsdom features which will facilitate testing fixes to submitter
serialization.

Note:
 * jsdom is making it harder to stub things, so we inject a jest-friendly
   window into the router. I'm not a huge fan of DI when used solely for the
   sake of testing, but this seems less terrible than other approaches (e.g.
   (patch-package-ing jsdom, or monkey-patching Object.defineProperty) 🙃
 * We use a yarn resolution to get the latest jsdom, despite the latest
   jest pinning it at ^20.0.0. Per the maintainers, this is fine as jest
   doesn't need any code changes to work with new jsdom, and there are no
   concrete plans yet to ship a major version.