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

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MR gitlab-foss!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 (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.

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

DarwinJS: Below I am suggesting using "Event" data that may already exist in the data turned over to the notification system.

Proposal

• Add a X-GitLab-NotificationReason header that contains the possible values from the user's notification settings (ex: watch, participate, on mention, custom)
• Add a X-GitLab-NotificationType header that contains the event that triggered the notification (ex: merged, new commit, merge conflict, ...)

Non-exhaustive list of notification types

• ....