Show it as successful pipeline, rather than success pipeline

Feedback:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6342#note_16132150

app/helpers/notifications_helper.rb

@@ -74,4 +74,13 @@ module NotificationsHelper
     return unless notification_setting.source_type
     hidden_field_tag "#{notification_setting.source_type.downcase}_id", notification_setting.source_id
   end
+
+  def notification_event_name(event)
+    case event
+    when :success_pipeline
+      'Successful pipeline'
+    else
+      event.to_s.humanize
+    end
+  end
 end

app/views/shared/notifications/_custom_notifications.html.haml

@@ -27,5 +27,5 @@
                       %label{ for: field_id }
                         = check_box("notification_setting", event, id: field_id, class: "js-custom-notification-event", checked: notification_setting.events[event])
                         %strong
-                          = event.to_s.humanize
+                          = notification_event_name(event)
                           = icon("spinner spin", class: "custom-notification-event-loading")