Bug 26225: Move translatable strings out of audio_alerts.tt and into audio_alerts.js

Bug 26225: Move translatable strings out of audio_alerts.tt and into audio_alerts.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 -> Audio alerts.
- Click the "Delete selected alerts" button without checking any
  checkboxes. You should see an error: "Check the box next to the alert
  you want to delete."
- Check the checkbox for an existing sound and click "Delete selected
  alerts." You should get a confirmation message, "Are you sure you want
  to delete the selected audio alerts?"
- Click "New alert."
- Without filling any details, click the "Play sound" button. You should
  see an error message, "Please select or enter a sound."

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 strings pulled from
  koha-tmpl/intranet-tmpl/prog/js/audio_alerts.js for translation, e.g.:

  msgid "Are you sure you want to delete the selected audio alerts?"
  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: David Nind <david@davidnind.com>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>