WebSearch: optional refersto/citedby record limit

• Adds optional record limit to refersto/citedby and similar second order operators, preventing possible DoS kind of attacks (e.g. when refersto:year:0->9999). (PR #2711)

• Adds configuration variable CFG_WEBSEARCH_MAX_RECORDS_REFERSTO and CFG_WEBSEARCH_MAX_RECORDS_CITEDBY.

• Limits result used by 'refersto', 'referstoexcludingselfcites' to CFG_WEBSEARCH_MAX_RECORDS_REFERSTO.

• Limits result used by 'citedby', 'citedbyexcludingselfcites' to CFG_WEBSEARCH_MAX_RECORDS_CITEDBY.

• Adds classes 'InvenioWebSearchReferstoLimitError' and 'InvenioWebSearchCitedbyLimitError'.

• Adds warning messages when the limit CFG_WEBSEARCH_MAX_RECORDS_* is reached.

• In BibRank adds 'record_limit' parameter to 'get_refersto_by_list', 'get_refersto_hitset', 'get_cited_by_list', 'get_citedby_hitset' (default to None).

• Adds regression test for the query 'refersto:year:2000'.

Signed-off-by: Federico Poli <federico.poli@cern.ch>
Reviewed-by: Samuele Kaplun <samuele.kaplun@cern.ch>
Reviewed-by: Tibor Simko <tibor.simko@cern.ch>