Expand the group info API endpoint
    
    The API endpoint can now include the list of projects this group has
    access to and even filter this list by the access level the group has
    on the projects.
    
    Fixes https://pagure.io/pagure/issue/2741
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr></pingou@pingoured.fr>