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>
This variable is user settable.  I think it should not be.  If `recstruct` is not exposable due to internal technical reasons, then we should either fix those reasons, or not allow admins to override our decision.  Please consider moving this variable into "internal" configuration file `search_engine_config.py` where only knowledgeable developers would experiment with it.

Alternatively, if we want to make this feature admin-configurable, then note that our formats are described in the `format` table already.  E.g. there is `visibility` column.  So we could introduce a special value like `-9` to denote internal formats that we don't expose.  Or we could add a new column.  This would have an advantage of centralising format-related settings in the same place.