cascaded access restrictions [7.6]

A lot of work tonight to implement automatic cascading of rights down the content hierarchy.

If you restrict access to a section, this will be propagated to sub-sections, articles, and related files, all at once.

Scalability is ensured up to 3,000 articles per section, but we can change the limit if necessary...

This new feature, who was asked for a while, simplifies a lot the implementation of sophisticated security policy.

Moreover, small icons are displayed to ensure that restricted access is always evident, if any.