Bug 23583: Handle OpacHiddenItems with yaml_preference

Bug 23583: Handle OpacHiddenItems with yaml_preference

This patch makes all places in Koha that rely on OpacHiddenItems
actually use C4::Context->yaml_preference instead of manually calling
the YAML libraries and handling it.

To test:
1. Apply this patch
2. Run:
   $ kshell
  k$ prove t/db_dependent/Items* \
           t/db_dependent/Koha/Item* \
           t/db_dependent/Koha/Filter/EmbedItems.t
=> SUCCESS: Tests pass!
3. Try hiding things with opac-ISBDdetail.pl and opac-search.pl
=> SUCCESS: Things work the same!
4. Sign off :-D

Signed-off-by: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>