Add a warning in email when "Reply by email" is disabled

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Problem to solve

When the Reply by email feature is not enabled, users who will receive emails from gitlab@gitlab.example.org, could not be aware that they are not supposed to reply to the email. Sometimes, error notification could take up to 2 days before the user who replied is notified about the fact.

Proposal

It would be great to add a warning on the top/bottom of each email, clearly stating that it is not possible to reply to the given email, and that replies should be entered by directly visiting the corresponding Issue/Merge request.

What does success look like, and how can we measure that?

No user will try to answer the email notifications sent by a "Reply by email"-disabled GitLab instance.