Use Shibboleth info instead of MAKACSESSION in SSO auth

[pferreir]

Indico currently suffers from a security issue:

1. Log in to Indico (through SSO)
2. Log in to another SSO service
3. Log out from said SSO service (one would expect to be logged out from all SSO services, including Indico)
4. Go back to Indico, still logged in

This is misleading for users, and can be problematic. The problem is that the authentication process is made in the following way:

1. Indico redirects user to SSO for login
2. User logs in
3. SSO redirects user to Indico, along with authentication tokens
4. Shibboleth parses tokens and provides Indico with user info
5. Indico attributes MAKACSESSION token to current session

The last step should be avoided, since it introduces the problem of revoking MAKACSESSION as soon as the current session doesn't possess a Shibboleth identity.
So, we should deal with the information that is provided by the authentication mechanism, instead of delegating identity into MAKACSESSION (in SSO auth).