Add a link to the group when viewing a namespace
    
    When viewing projects of a namespace, it is nice to be able to access
    quickly the group which is underneath this namespace (assuming there is
    one, ie that the namespace isn't in the list of allowed_prefix).
    
    Fixes https://pagure.io/pagure/issue/3582
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>