Collect number of claims, identifiers and sitelinks

[brawer]

The counters wb-claims, wb-identifiers and wb-sitelinks in the page_props table of wikidatawiki would make interesting signals for ranking. We should consider exporting them, together with additional ranking signals, in some signals.csv. (This probably won’t happen anytime soon, unless anyone wants to help out. But let’s keep it noted for later).

For example, the properties for the page www.wikidata.org/wiki/Q72 are:

INSERT INTO `page_props` VALUES
  (200,'kartographer_frames','1',1),
  (200,'page_image_free','Zuerich_Fraumuenster_St_Peter.jpg',NULL),
  (200,'wb-claims','550',550),
  (200,'wb-identifiers','85',85),
  (200,'wb-sitelinks','186',186);

[brawer]

We now compute this metric as part of item_signals.