Direct Single Sign On integration

## What?
Currently there is no Kerberos or SAML2 etc. implementation in Graylog to enable Single Sign On.
There only exists a SSO plugin (https://github.com/Graylog2/graylog-plugin-auth-sso) which depends on an additional/external webserver in front of graylog for SSO authentification.

## Why?
We dont want to install/maintain extra webservers only to enable SSO. Because graylog has its own webserver and you distribute an enterprise software, this feature should be mandatory.