Further refined the example: Simplified the setup

Further refined the example: Simplified the setup

* Goal: Do not rely on knowledge about Thymeleaf
* Goal: Separate MVC (controller) from business logic (service)
* Introduced an ExampleService
* The service checks the answer for
  The Ultimate Question Of Life, The Universe And Everything
* Requests without any answer are tolarated, so that the page with the
  question can be rendered
* Only numbers (Integer) are allowed as answer.
* Negative numbers are not allowed as answers: the service answers with an
  empty Optional.
* The view contains a bug, that results in a 503 for negative answers:
  Optional.get() is called, regardless of the state of the Optional

* The view renders the question, if no answer is specified (initial request!)
* If a valid request is specified, the answer and the outcome is rendered
* If an invalid request (negative number!) is specified, the view generates
  an exception, because it tries to resolve the Optional anyway