FIX: Strip Auto-Submitted email header from group SMTP emails (PR #15057)

Remove Auto-Submitted header for group private message emails, it does not make sense there and may hurt deliverability.

From https://www.iana.org/assignments/auto-submitted-keywords/auto-submitted-keywords.xhtml:

Indicates that a message was generated by an automatic process, and is not a direct response to another message.

GitHub

          TopicAllowedGroup.create!(topic: post.topic, group: group)