Resolve "Emails on Push are sent to only the first recipient when using AWS SES"

What does this MR do?

Ensures that we only set the Auto-Submitted and X-Auto-Response-Suppress headers a single time when the same mail object is delivered more than once.

Are there points in the code the reviewer needs to double check?

Don't think so!

Why was this MR needed?

Explanation in https://gitlab.com/gitlab-org/gitlab-ce/issues/39593#note_45083692.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #39593 (closed).