pppery, dreamyjazz: Backport for [[gerrit:1275112|Diqwiki: change project namespace (T328207)]], [[gerrit:1276025|Remove unused wgCheckUserUserAgentTableMigrationStage config]], [[gerrit:1276024|CheckUser Suggested Investigations: Enable on commonswiki (T424084)]] synced to the testservers (see https://wikitech.wikimedia.org/wiki/Mwdebug). Changes can now be verified there.