cubicweb #4919855 Implement security context [in-progress]

The idea is to be able to write security rql expressions that refer to

arbitrary values/entities, and not only 'U' for the current user.

This security context would be a dictionary in which keys could be used in permissions rql expressions.

Such a context would be very useful to check permissions against arbitrary informations orthogonal to the current user. For example, a token that gives access to a specific resource could be used to share a private resource via a simple url including this token.

Once such a system works, the current user (_cw.user) and its security related attributes (groups) could be transfered to this context, making cnx.user useless and less a problem than it is today [1].

Another big advantage would be that it would be easier to have an external system providing security informations without hacking around the user, connection and session [2]. One could even have permissions checking without a single CWUser in the database.