Checking uniqueness of the display_name before creating a group
    
    We have an unique constraint on the display name of a group, however,
    if you try creating a group with a display_name that is already taken,
    we do not inform you of this, making it hard to guess.
    This patch checks for the uniqueness of the display_name :)
    
    Also added the required unit-tests :)
    
    Fixes: https://pagure.io/pagure/issue/1633