Migrate ScrollDownButton to Storybook

Co-authored-by: Chris Svenningsen <chris@carbonfive.com>

ts/components/conversation/ScrollDownButton.stories.tsx

@@ -0,0 +1,38 @@
+import * as React from 'react';
+import { storiesOf } from '@storybook/react';
+import { action } from '@storybook/addon-actions';
+import { boolean } from '@storybook/addon-knobs';
+
+// @ts-ignore
+import { setup as setupI18n } from '../../../js/modules/i18n';
+
+// @ts-ignore
+import enMessages from '../../../_locales/en/messages.json';
+
+import { Props, ScrollDownButton } from './ScrollDownButton';
+
+const i18n = setupI18n('en', enMessages);
+
+const createProps = (overrideProps: Partial<Props> = {}): Props => ({
+  i18n,
+  withNewMessages: boolean(
+    'withNewMessages',
+    overrideProps.withNewMessages || false
+  ),
+  scrollDown: action('scrollDown'),
+  conversationId: 'fake-conversation-id',
+});
+
+const stories = storiesOf('Components/Conversation/ScrollDownButton', module);
+
+stories.add('No New Messages', () => {
+  const props = createProps();
+
+  return <ScrollDownButton {...props} />;
+});
+
+stories.add('New Messages', () => {
+  const props = createProps({ withNewMessages: true });
+
+  return <ScrollDownButton {...props} />;
+});