Issue #2970848 by quietone, maxocub: i18n Variable to config: site offline message [d7]

...nslation/migrations/d6_system_maintenance_translation.yml

...fig_translation/migrations/d6_system_site_translation.yml

...onfig_translation/migrations/d6_user_mail_translation.yml

...g_translation/migrations/d6_user_settings_translation.yml

...nslation/migrations/d7_system_maintenance_translation.yml

...el/Migrate/d7/MigrateSystemMaintenanceTranslationTest.php

...al/src/Plugin/migrate/source/d6/D6VariableTranslation.php

...upal/src/Plugin/migrate/source/d6/VariableTranslation.php

...upal/src/Plugin/migrate/source/d7/VariableTranslation.php

core/modules/migrate_drupal/tests/fixtures/drupal7.php

.../src/Kernel/Plugin/migrate/source/d6/i18nVariableTest.php

...rnel/Plugin/migrate/source/d7/VariableTranslationTest.php

...upal/tests/src/Unit/source/d6/VariableTranslationTest.php

...s/src/Functional/d7/MigrateUpgrade7NoMultilingualTest.php

...tests/src/Functional/d7/MigrateUpgrade7ReviewPageTest.php

...drupal_ui/tests/src/Functional/d7/MigrateUpgrade7Test.php