Setup routing within App.js

This commit modifies App.js to use routing, such that it's possible to
navigate between a home screen, and the Artifact Editor.

Test plan: Run `yarn start`, and navigate between the home screen and
artifact editor. Verify that the artifact editor still works as
expected.

src/app/App.js

@@ -1,10 +1,38 @@
 // @flow
 
 import React from "react";
+import {BrowserRouter as Router, Route, NavLink} from "react-router-dom";
+
 import ArtifactEditor from "../plugins/artifact/editor/App";
 
 export default class App extends React.Component<{}> {
   render() {
-    return <ArtifactEditor />;
+    const ARTIFACT_EDITOR_ROUTE = "/plugins/artifact/editor";
+    return (
+      <Router>
+        <div>
+          <nav>
+            <ul>
+              <li>
+                <NavLink to="/">Home</NavLink>
+              </li>
+              <li>
+                <NavLink to={ARTIFACT_EDITOR_ROUTE}>Artifact Editor</NavLink>
+              </li>
+            </ul>
+          </nav>
+
+          <hr />
+          <Route exact path="/" component={Home} />
+          <Route path={ARTIFACT_EDITOR_ROUTE} component={ArtifactEditor} />
+        </div>
+      </Router>
+    );
   }
 }
+
+const Home = () => (
+  <div>
+    <h1>Welcome to SourceCred! Please enjoy your stay.</h1>
+  </div>
+);