Send a notification upon editing the initial comment of a PR
    
    We're sending notification when someone edits the initial comment
    of a PR. This notification can now be using any of the bus we
    support or simply web-hooks.
    
    Fixes https://pagure.io/pagure/issue/4398
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr></pingou@pingoured.fr>