WebSubmit: file stamper option to copy metadata

* Introduces the "--skip-metadata" option to the file stamper.
  If enabled, the file stamper will not copy the PDF metadata from the input
  file to the (stamped) output file. Otherwise, the file stamper will attempt
  to copy all PDF metadata.  (closes #1569)

Signed-off-by: Nikolaos Kasioumis <nikolaos.kasioumis@cern.ch>
Acked-by: Tibor Simko <tibor.simko@cern.ch>