Make mail notifications of discussion notes In-Reply-To of each other

What does this MR do?

When a note is part of a discussion, the email sent out will be In-Reply-To the previous note in that discussion. It also References all the previous notes in that discussion, and the original issue.

Screenshots

 . [   5: GitLab.toon          ] Re: Gitlab Test | Some issue with discussions (#12)
 .     <   5: GitLab.toon          > 
 .     <   7: GitLab.toon          > 
 .         [   7: GitLab.toon          ] 
 .     <   7: GitLab.toon          > 
 .         [   7: GitLab.toon          ]

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