Issue #2913156 by Jaypan: Support for integration with message stack

..._display.message.private_message_notification.default.yml

..._display.message.private_message_notification.default.yml

...isplay.message.private_message_notification.mail_body.yml

...lay.message.private_message_notification.mail_subject.yml

....private_message_notification.field_message_pm_thread.yml

...te_message_notification.field_message_private_message.yml

...install/field.storage.message.field_message_pm_thread.yml

...l/field.storage.message.field_message_private_message.yml

...install/message.template.private_message_notification.yml

config/install/private_message.mail.yml

config/schema/private_message.schema.yml

private_message.info.yml

private_message.module

private_message.services.yml

src/Service/PrivateMessageNotifier.php

src/Service/PrivateMessageNotifierInterface.php

src/Service/PrivateMessageThreadManager.php