WebSearch: CERN-specific collection sorting

* Sorts 'CERN Courier Issues/Articles' and 'CERN Bulletin
  Issues/Articles' collections by 773__y field in WebColl.
  (closes #2017)

Signed-off-by: Sebastian Witowski <sebastian.witowski@cern.ch>
Reviewed-by: Tibor Simko <tibor.simko@cern.ch>