Make Indico compatible with CERN's new DPP

[pferreir]

There is a new CERN Data Protection Policy being drafted. This will imply that all CERN-stored data will have to respect a series of constraints, depending on its classification. Indico already does something like that, as content can be set as "public" or "restricted". However, other levels may be required.

Since we will have to eventually conform to this policy at CERN, it would be nice to find a simple and configurable scheme that allows administrators to cope with local security rules without necessarily enforcing any particular behavior.

[pferreir]

It would be also interesting to have an access level called "authenticated users" that would require authentication with any Indico account (stops bots, etc).