Bug 27070: Add cross_fields type to our searches

Bug 27070: Add cross_fields type to our searches

This patch adds the 'cross_fields' type to our searches:
https://www.elastic.co/guide/en/elasticsearch/reference/6.8/query-dsl-query-string-query.html#query-string-syntax

Without this patch the search terms seem to all require being in the same field when using Elasticsearch 6

To test:
0 - Set QueryAutoTruncate to 'only if * is added'
1 - Find a record with a title and publisher
2 - Search for a word form the title and confirm the record is returned
3 - Search for a work from the title and the publisher's name
4 - The record is not returned
5 - Apply patch
6 - Repeat #3
7 - The record is returned

Signed-off-by: Victor Grousset/tuxayo <victor@tuxayo.net>
Signed-off-by: Joonas Kylmälä <joonas.kylmala@helsinki.fi>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>