Bug 23797: Convert OpacLoginInstructions system preference to news block

Bug 23797: Convert OpacLoginInstructions system preference to news block

This patch builds on Bug 22318 to move the OpacLoginInstructions 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 OpacLoginInstructions system
preference. Apply the patch and run the database update process.

 - Go to the OPAC and click the login link.
 - In the login modal, confirm that the content which was previously in
   the OpacLoginInstructions system preference displays correctly
   below the login form.
 - While not logged in to the OPAC, navigate directly to
   /cgi-bin/koha/opac-user.pl. The OpacLoginInstructions content should
   display correctly here as well.
 - In the staff client, go to Tools -> News and verify that the content
   from OpacLoginInstructions is now stored in news items. There
   should be one entry for each of the enabled translations in your
   system, for instance 'OpacLoginInstructions_en',
   'OpacLoginInstructions_fr-FR', 'OpacLoginInstructions_cs-CZ'
 - Go to Administration -> System preferences and confirm that the
   OpacLoginInstructions preference has been removed.
 - To test the correct selection of language-specific content you must
   run the translation update/install process for the languages you're
   updating.

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>