Bug 19481: Set default sort options as sortable in ES default mappings

Bug 19481: Set default sort options as sortable in ES default mappings

To test:
1 - Setup Koha with ES
2 - perform a search
3 - try sorting by pubdate, acqdate, popularity, or title
4 - Search fails!
5 - Apply patch
6 - visit:
/cgi-bin/koha/admin/searchengine/elasticsearch/mappings.pl?op=reset&i_know_what_i_am_doing=1
7 - Re-index ES
8 - Perform search
9 - All sorting options should work

Signed-off-by: David Bourgault <david.bourgault@inlibro.com>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>