jupyter: Added a notebook extensions directory

software/jupyter/instance.cfg.in

@@ -100,6 +100,7 @@ jupyter_config_dir = ${:jupyter_dir}/etc
 jupyter_kernel_dir = ${:jupyter_dir}/kernels
 jupyter_runtime_dir = ${:jupyter_dir}/runtime
 jupyter_custom_dir = ${:jupyter_config_dir}/custom
+jupyter_nbextensions_dir = ${:jupyter_dir}/nbextensions
 erp5_kernel_dir = ${:jupyter_kernel_dir}/ERP5
 
 [jupyter_notebook]