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

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?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes gitlab-org/gitlab-ce#36054