Improve cold start performance

ts/util/getMessageTimestamp.ts

@@ -0,0 +1,8 @@
+// Copyright 2021 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+import { Message } from '../components/conversation/media-gallery/types/Message';
+
+export function getMessageTimestamp(message: Message): number {
+  return message.received_at_ms || message.received_at;
+}