Issue #3422872 by kunal.sachdev, Wim Leers: Add validation constraints to contact.settings

core/lib/Drupal/Core/Config/Schema/SchemaCheckTrait.php

core/modules/contact/config/schema/contact.schema.yml

core/modules/contact/contact.post_update.php

core/modules/contact/migrations/d6_contact_settings.yml

core/modules/contact/migrations/d7_contact_settings.yml

core/modules/contact/src/Controller/ContactController.php

...ules/contact/tests/src/Functional/ContactSitewideTest.php

...ntact/tests/src/Functional/Update/NullDefaultFormTest.php