[16.0][ADD] new module project_customer_access

[clementmbr]

Project menu and views for customers accessing your own ERP.

Module based on :

• cross_connect_server to make the connection between the two ERP
• base_group_backend to give restricted backend access to the customer.

Task "[18441] Refonte module support"

[clementmbr]

@sebastienbeau @Kev-Roche my project_customer_access is ready for review!

I just have a doubt about 2 things :

• I didn't find any modules displaying the cross_connect_client_id field on res.users form view, so I added it here for testing purposes through the UI... I understand it could be much better to put it in another module, but where?

• There is this strange non-obvious behavior with the UI when you want to create a user in a security group based on the "Backend User" group : as the "Backend User" group is from the "User Type" category, the UI doesn't display other categories if this first group is selected.
  The workaround is to add the user to the group through the group itself as I explained in the Readme. Let me know if it needs to be improved (if it's really needed to create these kind of customer users through the UI).

Thanks!