Add an example for Storybook, will be removed later

[MAILPOET-2769]

assets/js/src/storybook_demo/_stories/button.js

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+import React from 'react';
+import { action } from '@storybook/addon-actions';
+import { Button } from '@storybook/react/demo';
+
+export default {
+  title: 'Button',
+  component: Button,
+};
+
+export const Text = () => <Button onClick={action('clicked')}>Hello Button</Button>;
+
+export const Emoji = () => (
+  <Button onClick={action('clicked')}>
+    <span role="img" aria-label="so cool">
+      😀 😎 👍 💯
+    </span>
+  </Button>
+);

assets/js/src/storybook_demo/_stories/welcome.js

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+import React from 'react';
+import { linkTo } from '@storybook/addon-links';
+import { Welcome } from '@storybook/react/demo';
+
+export default {
+  title: 'Welcome',
+  component: Welcome,
+};
+
+export const ToStorybook = () => <Welcome showApp={linkTo('Button')} />;
+
+ToStorybook.story = {
+  name: 'to Storybook',
+};