allow `require("nbextensions/extname")` to load from IPYTHONDIR/nbextensions

IPython/html/templates/page.html

@@ -28,7 +28,10 @@
     <script src="{{static_url("components/requirejs/require.js") }}" type="text/javascript" charset="utf-8"></script>
     <script>
       require.config({
-          baseUrl: '{{static_url() }}',
+          baseUrl: '{{static_url()}}',
+          paths: {
+            nbextensions : '{{ base_project_url }}nbextensions'
+          }
       });
     </script>