Allow the creation of multiples studies

[nicoSix]

Right now, one deployment of Ardent can handle one study. This is enough for our project, but if we want this tool to be more useful later we must allow users to create multiple studies. This changes many things in user management:

• At the moment, a user can join Ardent using an invite code from an admin. But roles will have to be redefined : admin of a study, member of a study, member of Ardent, admin of Ardent ... ?
• If the system changes, allowing new users to register should be done using with emails/phones (and invite codes will be used maybe to join a study?)

This issue requires to:

• Redefine a new user management model for the application;
• Design the new interfaces;
• Implement the system, especially for the onboarding of users