ensure IPython._target is defined

IPython/html/static/base/js/namespace.js

@@ -5,5 +5,6 var IPython = IPython || {};
 define([], function(){
     "use strict";
     IPython.version = "3.0.0-dev";
+    IPython._target = '_blank';
     return IPython;
 });

IPython/html/static/tree/js/notebooklist.js

@@ -293,7 +293,7 define([
         // directory nav doesn't open new tabs
         // files, notebooks do
         if (model.type !== "directory") {
-            link.attr('target',IPython._target||'_blank');
+            link.attr('target',IPython._target);
         }
         if (model.type !== 'directory') {
             this.add_duplicate_button(item);