Bug 24220: Move OpacMoreSearches to HTML customizations in additional-contents

Bug 24220: Move OpacMoreSearches to HTML customizations in additional-contents

To test:
1- Before applying the patch add some content to the system preference
   OpacMoreSearches
2- Apply patch
3- restart_all and updatedatabase
4. Go to additional-content.pl ( HTML customizations )
5. You should now see your content from the system preference under
   OpacMoreSearches in HTML customizations
6. Try searching for the system preference, OpacMoreSeaches. It has been
   removed and you should not be able to find it.
7. Try changing/removing/adding content from OpacMoreSearches in HTML
   customizations
8. Make sure your edits are reflected when you look at the
   OpacMoreSearch area in the OPAC.

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>