Issue #2447727 by webflo, quietone, jofitz, Neslee Canil Pinto, mradcliffe, claudiu.cristea, k4v, yogeshmpawar, kgoel, dimaro, hussainweb, vprocessor, malcomio, iMiksu, phenaproxima, keithm, tlyngej, benjy, heddn, infopicard, Kristen Pol, KarenS: Add base class for migrating reference fields

Issue #2690001 by quietone, vasi, mikeryan, infopicard, phenaproxima: Migrate: Don't deduplicate forum vocabulary