What does this MR do?

By including the discussion IDs of indivudal note discussions, which are notes that can be turned into discussions but haven't yet, in the X-Gitlab-Discussion-ID header for mail clients, replies to individual notes will convert the note to a disucssion, preserving comment threading, instead of creating a top level note.

This makes an email reply look more like a reply than another top-level note.

Screenshots (strongly suggested)

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Does this MR meet the acceptance criteria?

Conformity

• Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team