Add the possibility to get the group members when asking the project info
    
    With this commit the /api/0/<project> API endpoint may include a
    ``group_details`` entry in its returned JSON blob which lists all the
    members of the groups linked to this project.
    
    Relates to: https://pagure.io/fedora-infrastructure/issue/6357
    Relates to: https://github.com/fedora-infra/bodhi/issues/1810
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>