When loading comment from JSON rely on username/comment rather than comment id
    
    Instead of relying on comment id which may vary from instance to instance
    depending on what is in the database, use the combo username/comment to
    find out if a comment was already made to the ticket (so that if not, it
    gets created).
    We are short-coming the case where the same user submits twice the exact
    same comment, so be it.
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>