Issue #1856562 by andypost | sun: Convert "Subject" and "Message" into Message base fields.

...ore/Field/Plugin/Field/FieldFormatter/StringFormatter.php

...upal/Core/Field/Plugin/Field/FieldType/StringLongItem.php

...e/Field/Plugin/Field/FieldWidget/StringTextareaWidget.php

.../config_translation/src/Tests/ConfigTranslationUiTest.php

core/modules/contact/contact.module

core/modules/contact/src/Entity/Message.php

core/modules/contact/src/MessageForm.php

...odules/contact/src/Tests/ContactAuthenticatedUserTest.php

core/modules/contact/src/Tests/ContactPersonalTest.php

core/modules/contact/src/Tests/ContactSitewideTest.php

...ules/text/src/Plugin/Field/FieldWidget/TextareaWidget.php