websearch: limit on refersto and simular keywords

* Limit DOS attacks with 'refersto' and similar keywords (e.g.
  'refersto:year:0-->9999').

* 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 'input_limits' 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>