Add rights on workers

• Update access rights to repositories
• Update endpoints concerning workers (versions, runs) derived from both rights on worker and its repository
• Migration to add a repository admin right to the owner of their credentials
• Allow the generic edition of repo/workers membeships
• Serialize access level in responses
• Serialize the members count on repositories (same behavior as corpora)
• Generic membership management endpoint should handle special right on workers (via a repository)