Clarify the access rights to projects for users (nuxeo, admin, studio_admin, etc.)

Clarify with Alain what are the rules for the different kind of users and groups, to access projects.

Implement it.

See:
UserServiceImpl.isUserMemberOfProject() Is it still needed? If the user had access to the project, he must have role or write access (admin or studio manager). (see NXS-3046)