Study how test center user and organizations work?

We should study how organizations or projects work for Test center. THis is needed because we will have only one test center installation for all customers. 

this also means that we should be able to

• create a project that is common for one Gitlab project or customer. 
• give access rights to conan_user:conan_password to this project
• hide this project from every other conan_user who does not have access to this project