refactor: Streamlined the API of the services

refactor: Streamlined the API of the services

- `ChatRoomService` and `ChatHomeService` both return only reactive types.
- The decission stems from the wish, to become reactive all the way from
  the client to the technical implementation of the backend-services.
- This faciliates the mapping of the results in `ChatBackendController`.
- The refactoring already simplified lots of code, where a `Flux` has
  to derived from the `Stream`, yielding a good feeling about the plan,
  that is pursued with this refactoring.
- The refactoring also lead to the decision that `UnknownChatroomException`
  realy is a business-logic-exception (that according refactoring was
  already performed in the previous commits, to ceap this commit clean).