Bug 23794: Convert OpacMainUserBlock system preference to news block

Bug 23794: Convert OpacMainUserBlock system preference to news block

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

Signed-off-by: Bernardo Gonzalez Kriegel <bgkriegel@gmail.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>