WIP: Make notes in discussions In-Reply-To of each other

Open Toon Claes requested to merge tc-correct-email-in-reply-to into master

What does this MR do?

When a note is part of a discussion, the email sent out should be In-Reply-To the previous note in that discussion.

Screenshots (TODO)

Does this MR meet the acceptance criteria?

What are the relevant issue numbers?

Closes gitlab-org/gitlab-ce#36054