Set X-GitLab-NotificationReason header if notification reason is explicit subscription
What does this MR do?
What the title says. :) This can be used in email filters to differentiate notification emails of stuff you have explicitly subscribed to.
Does this MR meet the acceptance criteria?
- Changelog entry
- Documentation created/updated or follow-up review issue created
- Code review guidelines
- Merge request performance guidelines
- Style guides
- Database guides
- Separation of EE specific content
Related to #34416 (closed)