Add jest to avoid causing regression errors

Jest can be added incrementally and made as part of a commit hook. This will allow for simple regression tests to be automated and ensure that we don't lose features as we build.

https://facebook.github.io/jest/docs/en/tutorial-react.html

No files or tests are named. Start by reading the linked Jest React tutorial and inspecting the repository's existing test and commit-hook setup; done means Jest is added incrementally, regression tests can run automatically, and the hook invokes them without losing existing features.

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