Bug 27252: Add deprecation warning, cross_fields pref, and add version to ES info

Bug 27252: Add deprecation warning, cross_fields pref, and add version to ES info

This patch prepares Koha to officially no longer support Elasticsearch 5.X

It adds a new system preference 'ElasticsearchCrossFields' to allow users to choose whether or not
to enable this feature

It updates the about page to add a deprecation warning if a site is running ES5

To test:
 1 - Be running Koha with Elasticsearch 5.X
 2 - Attempt to search
   Error: Unable to perform your search. Please try again.
 3 - Apply patch
 4 - Update database
 5 - Searching works
 6 - Find syspref 'ElasticsearchCrossFields'
 7 - Enable it
 8 - Searching is now broken
 9 - Check the about page
 10 - you can now see the Elasticsearch version
 11 - The systeminformation tab has a deprectaion warning
 12 - Set SearchEngine preference to 'Zebra'
 13 - View the about page - no warnings
 14 - Test again with ES6 - searching should "work" with either pref setting
 15 - There should be no warning on about pages

Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>