WebUI: Allow to add certs to certmapping with CERT LINES around
    
    The certificate to the certmapping might be inserted as
    base64 encoded blob. This patch allows to also insert the certificate
    blob with surrounding "-----BEGIN CERTIFICATE-----" and
    "-----END CERTIFICATE-----" lines. This behavior is the same in
    widget for assigning certificates to users, so the change helps
    WebUI to be more consistent.
    
    https://pagure.io/freeipa/issue/6772
    
    Reviewed-By: Florence Blanc-Renaud <frenaud@redhat.com>
    Reviewed-By: Petr Vobornik <pvoborni@redhat.com>