Set up testing framework for the API

[jstavanja]

Add Jest to the server application and set up a basic scaffolding for testing our API. We should use Jest as it is set up in the client Vue application as well.

We should also add a command, which runs both the API and client tests to use with CI.
The unit tests in the client are run with yarn test:unit or npm test:unit.

We can also add e2e tests to the client, as they are already set up. They are also run with yarn test:e2e or npm run test:e2e.

A basic Travis CI configuration that runs the tests would also be a great addition.

[jstavanja]

Added jest to the Node.js server application, but we should now try and add meaningful tests which may require some refactoring of our controllers.

We should allow dependency injection for the Impulze and User models so we can later mock them in our tests and just ensure that they are being called. We should also be able to create temporary databases so we don't run tests on the development database in which we store random data.