Add more notification reasons to the `X-GitLab-NotificationReason` email header (and email footer)
MR gitlab-ce!16160 implements a way to signal to the user why they have received an email notification, but only implements
mentioned reasons. The original issue (gitlab-ce#41532) has some more relevant discussion.
This issue is meant to encompass future work required in order to define all possible reasons for a user to be notified.
- Label subscription
- Watching the project, although not a participant in the topic of the email.
- Participant (has commented in the past or is issue owner)
We could also consider having a separate reason for the owner of the issue/mr/etc