The pages listed when browsing the /admin/pages/ explorer differed from
the pages listed in the new React/admin API powered pop-out explorer.
The latter did not pass the queryset through the
'construct_explorer_page_queryset' hook, so pages that should have been
hidden were visible. Visiting these pages in the explorer could then
lead to unexpected behaviours, as hidden sections of the site became
browsable.
A new `for_explorer=1` query parameter has been added to the admin API,
which will pass the page queryset through the
'construct_explorer_page_queryset' hooks.
