erp5: ERP5 and Jupyter integrated together

This patch series teaches ERP5 software release to automatically instantiate
Jupyter notebook web UI and tune it to connect to ERP5 by default. When Jupyter
is enabled, it also installs on-server erp5_data_notebook bt5 (see
nexedi/erp5!29 and nexedi/erp5@f662b5a2) which handles code execution requested
for Jupyter.

For ERP5 - for security and backward compatibility reasons - Jupyter
instantiation and erp5_data_notebook bt5 install happen only if jupyter is
explicitly enabled in instance parameters. The default is not to have Jupyter
out of the box.

On the other hand for Wendelin SR, which inherits from ERP5 SR, the
default is to have Jupyter out of the box, because Wendelin SR is fresh
enough without lots of backward compatibility needs, and Jupyter is
usually very handy for people who use Wendelin.

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NOTE Currently erp5-data-notebook bt5 has the following limitations (see
details on nexedi/slapos!43 and nexedi/erp5!29):

- errors are not reported properly to users;
- state is not fully saved to ZODB.

the latter point means notebook works only if it is connected to Zope family
with only 1 zope process. Hopefully this will be resolved some day.

Technical overview about how the integration is done itself on slapos part and
other notes are in 0a446263.

/proposed-for-review-on nexedi/slapos!43