WebSearch: add 'anyfield' key to CFG_WEBSEARCH_SYNONYM_KBRS

* CFG_WEBSEARCH_SYNONYM_KBRS can use the keyword 'anyfield' to
  match a given knowledge base when searching without a particular
  field. (closes #1493)

Signed-off-by: Jerome Caffaro <jerome.caffaro@cern.ch>
Reviewed-by: Ludmila Marian <ludmila.marian@cern.ch>