Issue #2470862 by mikeryan: First stage of UI - group listing page

config/install/migrate_plus.migration_group.default.yml

config/schema/migrate_plus.schema.yml

...mple/config/install/migrate_plus.migration_group.beer.yml

migrate_plus.module

migrate_tools/migrate_tools.links.action.yml

migrate_tools/migrate_tools.links.menu.yml

migrate_tools/migrate_tools.module

migrate_tools/migrate_tools.permissions.yml

migrate_tools/migrate_tools.routing.yml

migrate_tools/src/Controller/MigrationGroupListBuilder.php

migrate_tools/src/Form/MigrationGroupAddForm.php

migrate_tools/src/Form/MigrationGroupDeleteForm.php

migrate_tools/src/Form/MigrationGroupEditForm.php

migrate_tools/src/Form/MigrationGroupFormBase.php