Add more notification reasons to the `X-GitLab-NotificationReason` email header (and email footer)

MR !16160 (merged) implements a way to signal to the user why they have received an email notification, but only implements own_activity, assigned, and mentioned reasons. The original issue (#41532 (closed)) 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.

For example:

• 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)
• Subscriber

We could also consider having a separate reason for the owner of the issue/mr/etc

cc/ @smcgivern