Bug 23796: Convert OpacCustomSearch system preference to news block

Bug 23796: Convert OpacCustomSearch system preference to news block

This patch moves the OpacCustomSearch system preference into the Koha
news system, making it possible to have language- and library-specific
content.

To test you should have some content in the OpacCustomSearch system
preference. Apply the patch and run the database update process.

 - Go to the OPAC and confirm that the content which was previously in
   the OpacCustomSearch system preference now displays correctly where
   it was before.
 - In the staff client, go to Tools -> News and verify that the content
   from OpacCustomSearch is now stored in news items. There should be
   one entry for each of the enabled translations in your system, for
   instance 'OpacCustomSearch_en', 'OpacCustomSearch_fr-FR',
   'OpacCustomSearch_cs-CZ'
 - Go to Administration -> System preferences and confirm that the
   OpacCustomSearch preference has been removed.

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>