Set `X-GitLab-NotificationReason` header for email notifications due to explicit subscription

On notification emails, we set a header X-GitLab-NotificationReason. Currently, it conveys the information of whether the email is due to own activity, an assignment, or a mention. We should have it convey if the notification is due to user explicitly subscribing to an issue/MR/epic.

Explicitly subscribing usually means an increased interest in something, and it makes sense to filter such email notifications (from a flood of email notifications that user is getting due to other reasons). Having this header will help in that.