Add license headers across the project

ts/components/CallingButton.stories.tsx

@@ -1,3 +1,6 @@
+// Copyright 2020 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
 import * as React from 'react';
 import { storiesOf } from '@storybook/react';
 import { number, select } from '@storybook/addon-knobs';