Bug 23795: Convert opaccredits system preference to news block

Bug 23795: Convert opaccredits system preference to news block

This patch builds on Bug 22318 to move the opaccredits 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 opaccredits 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 opaccredits system preference now displays correctly where
   it was before.
 - In the staff client, go to Tools -> News and verify that the content
   from opaccredits is now stored in news items. There should be
   one entry for each of the enabled translations in your system, for
   instance 'opaccredits_en', 'opaccredits_fr-FR',
   'opaccredits_cs-CZ'
 - Go to Administration -> System preferences and confirm that the
   opaccredits preference has been removed.

Signed-off-by: Sally <sally.healey@cheshiresharedservices.gov.uk>
Signed-off-by: Alex Arnaud <alex.arnaud@biblibre.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>