As a system administrator, I want the startup command to automatically create the main workspace and first superuser, so that I can start the system without manual operations

[frankie567]

Plan of the changes:

• Add fief_admin_email setting. It's optional, with a default to None.
• Add fief_admin_password setting. It's optional, with a default to None.
• In the run_server command:
  • Check if the main workspace exists. If not, create it.
  • If fief_admin_email is not None, check if a corresponding user exists.
    • If not, create it. If fief_admin_password is None, generate a random password.