List group members
Introduce at project admin level, for each static AND dynamic (project_member, project_admin, etc) group a "permission/visibilty" to identify who is allowed to see and list one group.
For instance:
- Group Name: Developers
- Visibility: ["Authenticated", "Registered", "Project Members", ...]
The visibility list is built with the same data than Git and Mediawiki permissions.
This new visibility is applied on
- GET /v1/user_groups/{id}
- GET /v1/projects/{id}/user_groups