Issue #3096101 by quietone, danflanagan8, abhisekmazumdar, anmolgoyal74, alexpott, gabesullice, Wim Leers, Matroskeen, ankithashetty, mikelutz, benjifisher, daffie, webchick: Allow migrate_drupal_ui source database to be set in settings.php (cherry picked from commit 9fbab426)
