HTTP header authentication

The goal is to be able to log in trough HTTP Header, which is the default SSO method used by LemonLDAP::NG.

This is different from Basic authentication as we don't provide the password, only the user identifier (as we do with CAS).

I will provide a patch through github for this feature.

I would maybe need some help to add some config parameters in FD core schema to be compliant with upgrade process. Any hint?

(from redmine: issue id 5096, created on 2016-08-23, closed on 2016-09-07)