[FIX] website_slides: do not allow fullscreen to bypass ACLs

Currently fullscreen takes all information from categorized slides to
display its menu. In order to avoid calls to server some information
is prepared in DOM to speedup loading. It means slide information is
available even when not being member of a course which leads to some content
leak.

This commit fixes that by correctly checking that a slide can be accessed
before allowing to have access to its information and embedded code. Access
of a slide is either member of a course, either course publisher.

Task 2058595 (eLearning v13 testing)
Task 2064112 (fullscreen bug report)