Issue #2826021 by hchonov, Berdir, mkalkbrenner, plach, james.williams, tstoeckler: FieldItemList::equals is sufficient from the storage perspective but not for code checking for changes

Issue #2878556 by plach, matsbla, vijaycs85, Gábor Hojtsy, catch, gabesullice, effulgentsia, hchonov, hass, amateescu, xjm: Ensure that changes to untranslatable fields affect only one translation in pending revisions