Set a self reference in initial notification

What does this MR do and why?

Set a self reference in initial notification

This allows mail clients to display in the same thread initial notification and further responses even when using a mail provider

fix #215666

References

#215666 and #444279

How to set up and validate locally

In the Gitlab configuration configure the smtp server with a mail provider like Mailjet who is overriding the Message-ID Open an Issue. Then add a comment later. The two mail notifications should be in the same mail thread

Merge request reports

Loading