Try out End-2-End tests

[schnapster]

Let's try adding some E2E tests.

To be able to get into the parts behind the login we will probably need access to the java code so set up some mocks, so we would need a web ui testing library for Java. Seems like there aren't many around as I've only found https://github.com/selenide/selenide so far.

Let's give it a spin, write a few tests, and evaluate if it's worth it.