Bug 27360: Use 'public' to filter libraries for opac display

Bug 27360: Use 'public' to filter libraries for opac display

This patch adds the ability to set a library as 'public' or not; this
allows librarians to hide back office library branches from the opac.

Test plan
1/ Set a library as public from `admin > libraries`.
2/ Navigate to the `Libraries` page in the OPAC
3/ Note that only `public` libraries are displayed.

Signed-off-by: Barbara Johnson <barbara.johnson@bedfordtx.gov>
Signed-off-by: Christopher Brannon <cbrannon@cdalibrary.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>