Coverage badge misrepresenting the actual coverage

[baratgabor]

The coverage is currently only calculated for the application core, i.e. Application and Domain layers.

This is still highly meaningful, since that covers the actual business operations, but the WebApi and the Infrastructure assembly is excluded, as well as the Angular frontend assembly (WebUI).

Edit: Added unit tests with high coverage for WebApi.

Would be good to communicate this somehow, because currently it can potentially mislead people about the quality of the project, while the frontend in particular has zero testing.

(Sadly Coveralls badges don't provide customizability.)