Fix the URLs when opening a new PR vias the drop-down button
    
    Basically, in this commit we make sure to give a value to the branch_to
    and branch_from variable so the url is properly generated, then we remove
    the random value and replace them by the content returned by the ajax
    query.
    
    Fixes https://pagure.io/pagure/issue/3535
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>