Return the flag UID when adding or updating a flag on a PR not in fedmsg
    
    The UID of a flag is what allows to update it, so it shouldn't be part
    of the fedmsg message we sent.
    However, if the user is not specifying the UID, then they have no way to
    know it unless we return it, so when adding or updating a flag, return
    the JSON representation of that flag object.
    
    Adjust the unit-tests accordingly.
    
    Fixes https://pagure.io/pagure/issue/1588
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr></pingou@pingoured.fr>