Rename Storybook modules (#8112)

The names for Storybook modules have changed in v5.3.0. `config.js` was
renamed to `preview.js`, and addon registration has been moved to
`main.js`.

See here for details: https://github.com/storybookjs/storybook/blob/next/MIGRATION.md#from-version-52x-to-53x

.storybook/addons.js

@@ -1,2 +0,0 @@
-import '@storybook/addon-knobs/register'
-import '@storybook/addon-actions/register'

.storybook/config.js

@@ -1,11 +0,0 @@
-import { configure } from '@storybook/react'
-import '../ui/app/css/index.scss'
-
-const req = require.context('../ui/app/components', true, /\.stories\.js$/)
-
-function loadStories () {
-  require('./decorators')
-  req.keys().forEach((filename) => req(filename))
-}
-
-configure(loadStories, module)

.storybook/main.js

@@ -0,0 +1,7 @@
+module.exports = {
+  stories: ['../ui/app/components/**/*.stories.js'],
+  addons: [
+    '@storybook/addon-knobs',
+    '@storybook/addon-actions',
+  ],
+}