WebSearch: blacklist formats from search

* Introduces a new configuration variable CFG_WEBSEARCH_BLACKLISTED_FORMATS
  It lists all the invenio internal formats that should not be exposed by
  perform_request_search.
  (closes #1712)

Reviewed-by: Samuele Kaplun <samuele.kaplun@cern.ch>
Signed-off-by: Alessio Deiana <alessio.deiana@cern.ch>