Bug 26230: Move translatable strings out of item_search_fields.tt and into item_searc...

Bug 26230: Move translatable strings out of item_search_fields.tt and into item_search_fields.js

This patch removes the definition of translatable strings out of
templates and into the corresponding JavaScript file, using the new JS
i81n function.

To test:

- Apply the patch, go to Administration -> Item search fields.
- If necessary, add a new search field.
- From the table of search fields, click the "Delete" button for one of
  the fields.
- You should get a confirmation: "Are you sure you want to delete this
  field?"

TESTING TRANSLATABILITY

- Update a translation, e.g. fr-FR:

> cd misc/translator
> perl translate update fr-FR

- Open the corresponding .po file for JavaScript strings, e.g.
  misc/translator/po/fr-FR-messages-js.po
- Locate the string pulled from
  koha-tmpl/intranet-tmpl/prog/js/item_search_fields.js for translation:

  #: koha-tmpl/intranet-tmpl/prog/js/item_search_fields.js:18
  msgid "Are you sure you want to delete this field?"
  msgstr ""

- Edit the "msgstr" string however you want (it's just for testing).
- Install the updated translation:

  > perl translate install fr-FR

- Switch to your newly translated language in the staff client
  and repeat the test plan above. The translated strings should
  appear.

Signed-off-by: Alexis Ripetti <alexis.ripetti@inLibro.com>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>