Bug 25054: Display search field aliases in Search Engine Configuration

Bug 25054: Display search field aliases in Search Engine Configuration

It'd be great if the Search Engine Configuration page would display
the various aliases (shortcuts) available : ti for title, sn for local-number, etc.

Patch changes Koha/SearchEngine/Elasticsearch/QueryBuilder.pm to move
hard-coded vars at the beging and adds a method to provide to %index_field_convert via a method.

Test plan :
1) Use Elasticsearch
2) Go to Administration > Search engine configuration (Elasticsearch)
3) Check you see new column 'Aliases' with for example ti for title.
4) Perform a search 'ti:<title>' and check you get results

Signed-off-by: Séverine QUEUNE <severine.queune@bulac.fr>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>