Add Rake task to migrate timeline events

We're going to rename the `state_changed` event type to
`status_changed`.

lib/tasks/timeline_events.rake

@@ -0,0 +1,8 @@
+namespace :timeline_events do
+  desc "Migrate state_changed events"
+  task migrate_state_changed: :environment do
+    TimelineEvent.where(event_type: "state_changed").update_all(
+      event_type: "status_changed",
+    )
+  end
+end