setup React Storybook; add example story for LoadingIndicator component

storybook/config.js

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+import { configure } from '@kadira/storybook';
+import React from 'react';
+import { storiesOf, action } from '@kadira/storybook';
+
+window.storiesOf = storiesOf;
+window.action = action;
+window.React = React;
+
+function loadStories () {
+  require('./stories/loading_indicator.story.jsx');
+  // You can require as many stories as you need.
+}
+
+configure(loadStories, module);

storybook/stories/loading_indicator.story.jsx

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+import LoadingIndicator from '../../app/assets/javascripts/components/components/loading_indicator.jsx'
+
+storiesOf('LoadingIndicator', module)
+  .add('default state', () => (
+    <LoadingIndicator />
+  ));