Adds `#build_notification_recipients` to `NotificationRecipientService`
that returns the `NotificationRecipient` objects in order to be able to
access the new attribute `reason`.
This new attribute is used in the different notifier methods in order to
add the reason as a header: `X-GitLab-NotificationReason`.
Only the reason with the most priority gets sent.
factor out #suitable_notification_level? and check manually by
notification level. this makes the notification logic clear and actually
reflect what is in the documentation as to what should happen with each
setting.
Bob Van Landuyt