Further locale support for Javascript files (#3402)

* Add js-gettext. Works, but dirty.

* Updated gettext to get strings from a messages.json file.

* Added grunt task for updating translations.

* Tiny change

* Update Gruntfile.js

Switched from grunt-run to grunt-exec but forgot to change the tasks.

* Fix indents.

* A more elegant static path for locale: locale/<lang_code>.<file_type}