Issue #2976889 by phenaproxima, xjm, samuel.mortenson, chr.fritsch: Add a trait to create media types in tests and replace MediaFunctionalTestCreateMediaTypeTrait

Issue #2702683 by Manuel Garcia, xjm, prics, pguillard, Jo Fitzgerald, vg3095, ztl8702, lucian.cazaciuc, bojanz, tstoeckler, hchonov, rszrama, rteijeiro, svjuly, jibran, vdanielpop, vasike, jmikii, jhodgdon, Wim Leers, amateescu, Gabr1el, Drupa1ish, ciprian.stavovei: Add plural labels to entity types

Issue #2935951 by heddn, maxocub, quietone, Jo Fitzgerald, phenaproxima, alexpott, larowlan: Copy migrate source plugin from migrate_drupal_d8 into migrate_drupal