Client creation and edition

@sylvain : Do we really want to create a Client model ? Don't we want to include the Client data within the project's data ?

This will lead to duplicates, but my fear is that working for ENGIE La Défense T1 is not the same as working with ENGIE CRIGEN and that, eventually, it will be a lot of hassle on coding for very little to no benefit for the end user.

What do you think ?