Adds system preference to enable Jupyter kernel.

Thanks to this Jupyter code can easily become default ERP5 one and only after being configured by site user will expose potentially dangerous functionality.

By default Jupyter integration is disabled and site user with a Manager role is responsible to enable it. This is different from current state, when installing the erp5_data_notebook BT5 allows to remotely execute dangerous code.

TODO:

• encapsulate the test into Python Script assertion ERP5Site_assert...Enabled
• test that assertion follows preference
• mock assertion in tests