Bug 28125: Display a warning on the about page for OPACHiddenItems is needed

Bug 28125: Display a warning on the about page for OPACHiddenItems is needed

If OPACHiddenItems is containing badly construct it will lead to
inconsistent behaviours. For instance the holds view will not display
any items, when the detail page will display all of them (because we are
not using the same code, we are in transition to merge it).

This patch suggests to add a warning to the about page in case the query
generated by Koha::Items->filter_by_visible_in_opac produces a warning

Signed-off-by: Andrew Fuerste-Henry <andrew@bywatersolutions.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Bug 28125: (QA follow-up) Fix missing filter

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>