Bug 10595: don't display OpacTopissue page when system preference is turned off

Bug 10595: don't display OpacTopissue page when system preference is turned off

When the syspref is turned off, the link is hidden but the page is still
accessible. This patch checks the syspref and redirects to 404 if it is disabled.

Test plan:

Before applying the patch

Enable OpacTopissue
  Observe there is a link to opac-topissues.pl next to Advanced search etc.
  Follow that link, observe that opac-topissues.pl is loading
Disable OpacTopissue
  Observe the link is gone
  Manually open /koha/opac-topissues.pl and check that it is loading even though the feature is disabled

Apply patch

(OpacTopissue still diabled)
Manually open /koha/opac-topissues.pl and observe that you are being redirected to the 404 page.
Enable OpacTopissue
  Follow the link or manuall open opac-topissues.pl and check that it is working

Signoff the patch.

Signed-off-by: Srdjan <srdjan@catalyst.net.nz>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>