Bug 19482: Add support for defining 'mandatory' mappings

Bug 19482: Add support for defining 'mandatory' mappings

To test:
1 - Apply patch
2 - ./installer/data/mysql/updatedatabase.pl
3 - Reset ES mapping: Administration->Search engine configuration , button at bottom of page
4 - 'issues' and 'title' mapping under 'search fields' should be mandatory and not editable
5 - On 'Bibliographic records' tab you should not be able to delete the single entry for issues
6 - You should be able to delete 'title' mappings, however, at the final one you should be stopped by javascript
7 - Bonus: force remove the last mapping from the page using developer tools - attempt to save and should be warned of missing mandatory mapping

Signed-off-by: Nicolas Legrand <nicolas.legrand@bulac.fr>
Signed-off-by: Bouzid Fergani <bouzid.fergani@inlibro.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>