WebSearch: field-filtered MARCXML API output

* Implements field-filtered MARCXML output in Python and Web APIs.
  This was working for the TextMARC output, not for MARCXML output.
  This commit fixes the problem.  Usage: `/record/123?of=xm&ot=100,700`
  or `/search?p=ellis&of=xm&ot=100,700`.  (closes #1591)

* Adds new tests for trying to access hidden fields via the
  filtered-field API technique.

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
WebSubmit: optional pdftk regression tests

* Disables WebSubmit stamping related regression tests
  in case pdftk is not available.

Reported-by: Jiri Kuncar <jiri.kuncar@cern.ch>
Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>
Reviewed-by: Tibor Simko <tibor.simko@cern.ch>