WebSearch: limit DOS attacks with 'refersto' and similar keywords

* Adds configuration variable CFG_WEBSEARCH_MAX_RECORDS_REFERSTO
* Adds configuration variable CFG_WEBSEARCH_MAX_RECORDS_CITEDBY
* Limits the size of the intbitset used by the keywords 'refersto' and
  'referstoexcludingselfcites' to CFG_WEBSEARCH_MAX_RECORDS_REFERSTO
* Limits the size of the intbitset used by the keywords 'citedby' and
  'citedbyexcludingselfcites' to CFG_WEBSEARCH_MAX_RECORDS_CITEDBY