Bug 20388: Elasticsearch - Ability to add search fields from UI

Bug 20388: Elasticsearch - Ability to add search fields from UI

Test plan:

  - apply this patch,
  - check that SearchEngine system preference is set to Elasticsearch,
  - go to Admin > Search engine configuration,
  - on the search fields tab, fill a new line at the bottom of the table
    (name, label, type)
  - click on the "Add" button and save,
  - check that the new search field has been saved,
  - also test field deletions,
  - check that you can't delete already mapped fields.

Signed-off-by: Nicolas Legrand <nicolas.legrand@bulac.fr>
Signed-off-by: Séverine QUEUNE <severine.queune@bulac.fr>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Björn Nylén <bjorn.nylen@ub.lu.se>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Michal Denar <black23@gmail.com>
Signed-off-by: Michal Denar <black23@gmail.com>
Signed-off-by: Michaela Sieber <michaela.sieber@kit.edu>
Signed-off-by: David Schmidt <davewood@gmx.at>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Katrin Fischer <katrin.fischer@bsz-bw.de>