Bug 30873: Add system preference to hide libraries link in the OPAC

Bug 30873: Add system preference to hide libraries link in the OPAC

Some users would prefer that the libraries link not appear in the menu
of links under the OPAC's main search bar. This patch adds a preference
to do so.

The preference defaults to "On" since "On" is the current default
behavior.

To test, apply the patch and run the database update prcoess.

- In the OPAC, confirm that by default the "Libraries" link appears.
- In the staff interface, go to Administration -> System preferences ->
  OPACShowLibraries
- Set the preference to "Don't show"
- Return to the OPAC and confirm that the "Libraries" link isn't there.
- Try to navigate directly to /cgi-bin/koha/opac-library.pl. You should
  be redirected to a 404 page.

Sponsored-by: Athens County Public Libraries
Signed-off-by: Caroline Cyr La Rose <caroline.cyr-la-rose@inlibro.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>