Download as always starts downloads in new window/tab

This is a slightly worse user experience if it succeeds, because the new
tab flashes up before closing again, but it will let us display an
informative error page if it fails, without navigating the user away
from the interactive notebook view.

IPython/html/static/notebook/js/menubar.js

@@ -83,11 +83,7 b' var IPython = (function (IPython) {'
             notebook_name + '.ipynb'
         ) + "?download=" + download.toString();
 
-        if (download) {
-            window.location.assign(url);
-        } else {
-            window.open(url);
-        }
+        window.open(url);
     }
 
     MenuBar.prototype.bind_events = function () {