Use double quotes in CoffeeScript files

As we do in the rest of the application.

Note we cannot add a rule enforcing double quotes because CoffeeScript
Lint does not have such rule.

app/assets/javascripts/annotatable.js.coffee

@@ -2,18 +2,18 @@ _t = (key) -> new Gettext().gettext(key)
 
 App.Annotatable =
   initialize: ->
-    current_user_id = $('html').data('current-user-id')
+    current_user_id = $("html").data("current-user-id")
     if current_user_id == ""
       annotator.ui.editor.Editor.template = [
         '<div class="annotator-outer annotator-editor annotator-hide">',
         '  <form class="annotator-widget">',
-        "    #{_t('Unregistered')}",
+        "    #{_t("Unregistered")}",
         '    <div class="annotator-controls">',
-        "      <a href='#cancel' class='annotator-cancel'>#{_t('Cancel')}</a>",
-        '    </div>',
-        '  </form>',
-        '</div>'
-      ].join('\n')
+        "      <a href='#cancel' class='annotator-cancel'>#{_t("Cancel")}</a>",
+        "    </div>",
+        "  </form>",
+        "</div>"
+      ].join("\n")
 
     $("[data-annotatable-type]").each ->
       $this       = $(this)