Tests, tests, tests!

Since the project was created for internal use and quickly bootstrapped with `create-react-app` I did not give much thought to writing tests.

There are currently a couple of tests, but they basically check that the app doesn’t crash right away.

It would be great to cover the current code with tests and adopt a TDD development process for future features.

Please submit any meaningful tests you can think of for the current code, and I will gladly merge them!