Allow filtering user's PR by time information
    
    This commit adds support for filtering the PR filed or actionable for an
    user using time information such as the created date, the updated date
    or the closed date.
    The format supported it since..until. If the string provided does not
    include a '..', it is assume that only the since value was specified.
    
    Fixes https://pagure.io/pagure/issue/4065
    
    Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>