Add unittests

[ghost91-]

Summary

We should add unittests.

Description

As a developer, I want the code to be tested by unitetests so that I can rely on it working correctly.

Acceptance criteria

• If possible, functions should have unittests (stuff in calendarwebapp.calendarwebapp can not reasonably be tested and its probably also not usefull for the thin database wrappers).
• We should aim for a test coverage above 70%.