Create preflight welcome screen

To open the preflight web mode, we require basic auth. The password is autogenerated and provided in graylog-server logs. This is, judging by some bug reports, quite unexpected move.

We could simplify this step by providing a welcome screen without any auth, that would tell users that the graylog server started, it's now in the preflight mode and to configure it, you need to use username and password provided in the server logs. The raw basic-auth dialog is not really a welcoming and user-friendly approach.