[FEATURE] added authentication entry point

Introduced for delivering a 401 http code from the API, so that we can redirect to the login page, once the session expired.

__Tasks:__

- [ ] gather feedback for my changes

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | yes
| Fixed tickets | fixes https://github.com/sulu-cmf/sulu-standard/issues/110
| BC Breaks     | Returns a 401 instead of a 302, when the session in the api expired
| Doc           | none
I think AJAX-Requests should also get a 401 response.