Wikidata deployment

Deployment notes

 * purge the parser cache by bumping $wgCacheEpoch
 * make sure wikidata-shortcopyright message is handled properly ✅
 * add new data rights url / text settings
 * schema updates - check that they don't depend on deployment... i think they can be done before/after/whatever
 * https://gerrit.wikimedia.org/r/#/c/99637/ - (bug 47135) Make row IDs use BIGINT, db schema change, see repo/sql/MakeRowIDsBig.sql
 * https://gerrit.wikimedia.org/r/#/c/99660/ - (bug 45529) use composite indexes on wb_terms, db schema change, see repo/sql/UpdateTermIndexes.sql
 * old client code compatible with new repo code - with cache key varying on $wmfExtendedVersionNumber, should be fine. ✅
 * stop/update cron jobs to use code in new location
 * update config to load new extension-list-wikidata / old list conditionally

Deployment notes

 * add sites table and populate for wikisource

Changes

 * data type in json / api output
 * table of contents
 * sorting
 * ranks

Other

 * improvements to the link item widget in the client
 * using parser cache now
 * improvements to page loading in backend

Bug fixes

 * diff=0 works - 49434