Fixed book-tree-gen page visibility issue

Fixed book-tree-gen page visibility issue

When book trees were generated, pages in chapters where ALL pages within
were not supposed to be visibile, would be visible due to the code
falling back on the raw relation which would not account for
permissions.

This has now been changed so that a custom 'visible_pages' attribute is set and used by any book tree structures, to ensure it does not fall back to the raw relation.

Added an extra test to cover.

For #2414