Issue #2914974 by quietone, rakesh.gectcr, maxocub, heddn, Jo Fitzgerald, Yogesh Pawar, masipila, phenaproxima, jhodgdon, Gábor Hojtsy, larowlan, neclimdul: Migrate UI - handle sources that do not need an upgrade

Issue #2927844 by Jo Fitzgerald, quietone, heddn: Correct references to 'iterator' plugin to be 'sub_process'

Issue #2859304 by quietone, heddn, Jo Fitzgerald, maxocub, rivimey, phenaproxima, jhodgdon: Show field type migrations correctly in Migrate Drupal UI

Issue #2896507 by heddn, maxocub: Update FieldPluginBase with a default processFieldValues() and getFieldFormatterMap()