Also updates all jest related dependencies to be compatible with
jest@26.

Earlier versions of jest@26 had severe performance impacts, but that has
been resolved with jest@26.5. [0]

Thanks to this update `jest-environment-jsdom-sixteen` can be replaced
with the upstream `"jest-environment-jsdom` which is also based on
jsdom@16.

[0]: https://github.com/facebook/jest/issues/9457#issuecomment-703525151