bug: fix accessibility warning when open a Modal

add the following code:

  componentWillMount() {
    Modal.setAppElement('body');
  }

To prevent this accessibility warning in the console, whenever the Modal opens,

> Warning: react-modal: App element is not defined. Please use `Modal.
	setAppElement(el)` or `set appElement={el}`. This is needed so
	screen readers don't see main content when modal is opened. It is
	not recommended, but you can opt-out by setting `ariaHideApp={false}`

react-modal issue-133 describes the issue, and solutions are provided
	in the discussion:
	reactjs/react-modal#133
I also summarized these solutions on StackOverflow:
	https://stackoverflow.com/questions/48269381/warning-react-modal-app-element-is-not-defined-please-use-modal-setappeleme/48270342#48270342
And submitted github-issue/waffle item to Udacity:
	https://github.com/udacity/reactnd-issues/issues/233
Also in the process to updating the documents so I can submit a PR to
	for related issue-576:
	reactjs/react-modal#576

src/components/App.js

@@ -21,6 +21,10 @@ class App extends Component {
     ingredientsModalOpen: false,
   }
 
+  componentWillMount() {
+    Modal.setAppElement('body');
+  }
+
   openFoodModal = ({ meal, day }) => {
     this.setState(() => ({
       foodModalOpen: true,