Added email threading for update emails on issues and merge requests
This implements the following feature request: http://feedback.gitlab.com/forums/176466-general/suggestions/5168316-email-threading
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
This implements the following feature request: http://feedback.gitlab.com/forums/176466-general/suggestions/5168316-email-threading