Add license headers across the project

sticker-creator/index.tsx

@@ -1,3 +1,6 @@
+// Copyright 2019-2020 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
 import * as React from 'react';
 import { render } from 'react-dom';
 import { Root } from './root';