Add Rake script to backfill existing users' legacy users from `old_user_id`

lib/tasks/sync_legacy_users.rake

@@ -0,0 +1,11 @@
+namespace :core do
+  # TODO: Remove once ran on all environments.
+  desc "Creates a LegacyUser object for any existing Users"
+  task sync_legacy_users: :environment do
+    User.where.not(old_user_id: nil).includes(:legacy_users).find_each do |user|
+      next if user.legacy_users.where(old_user_id: user.old_user_id).any?
+
+      user.legacy_users.create!(old_user_id: user.old_user_id)
+    end
+  end
+end