Include datepicker i18n only as needed

* Restore the javascript_tag_for_i18n_datepicker helper (and actually use it)
* Include the datepicker i18n JS in the asset precompilation list

app/assets/javascripts/tracks_pages.js

@@ -111,7 +111,6 @@ var TracksPages = {
     TodoItems.setup_autocomplete_for_predecessor();
   },
   setup_datepicker: function() {
-    $.datepicker.setDefaults($.datepicker.regional[i18n_locale]);
     $('input.Date').datepicker({
         'dateFormat': dateFormat,
         'firstDay': weekStart,