Add Rake task to migrate timeline events

Since we're going to stop using the old_state and new_state fields and
instead rely on the existing old_value and new_value fields.

lib/tasks/timeline_events.rake

@@ -0,0 +1,15 @@
+namespace :timeline_events do
+  desc "Migrate old_state/new_state"
+  task migrate_old_new_state: :environment do
+    TimelineEvent
+      .where(event_type: %w[state_changed assessment_section_recorded])
+      .each do |timeline_event|
+        timeline_event.update!(
+          old_value: timeline_event.old_state,
+          new_value: timeline_event.new_state,
+          old_state: "",
+          new_state: "",
+        )
+      end
+  end
+end