Rename SlackNotificationService back to SlackService
What does this MR do?
See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8191#note_20310845
Do not update/delete: Banner broadcast message test data
Do not update/delete: Notification broadcast message test data
See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8191#note_20310845