WebUI: change validator of page size settings
    
    Previously, this configuration field was validated by integer_validator
    which only checks that the input is number.
    Now new positive_integer_validator can also check that
    the inputed number positive.
    
    https://pagure.io/freeipa/issue/6980
    
    Reviewed-By: Felipe Volpone <felipevolpone@gmail.com>