Block API Browser UI if current user is not logged in

[patrickmann]

The API Browser UI is available at a well-known URL, e.g.: https://graylog-test43.dev.torch.sh/api/api-browser/global/index.html

While you cannot call any of the APIs without being logged in (with the necessary permissions), we should not even render the UI if the user is not logged in.

[boosty]

We could implement a redirect to the normal Graylog login page.