Setup credExplorer app scaffold (#249)

Test plan: Run `yarn start`, and observe that the Cred Explorer is now
included in the nav bar, and can be selected, in which case a short
message is displayed.

src/app/App.js

@@ -4,10 +4,12 @@ import React from "react";
 import {BrowserRouter as Router, Route, NavLink} from "react-router-dom";
 
 import ArtifactEditor from "../plugins/artifact/editor/App";
+import CredExplorer from "./credExplorer/App";
 
 export default class App extends React.Component<{}> {
   render() {
     const ARTIFACT_EDITOR_ROUTE = "/plugins/artifact/editor";
+    const CRED_EXPLORER_ROUTE = "/explorer";
     return (
       <Router>
         <div>
@@ -16,6 +18,9 @@ export default class App extends React.Component<{}> {
               <li>
                 <NavLink to="/">Home</NavLink>
               </li>
+              <li>
+                <NavLink to={CRED_EXPLORER_ROUTE}>Cred Explorer</NavLink>
+              </li>
               <li>
                 <NavLink to={ARTIFACT_EDITOR_ROUTE}>Artifact Editor</NavLink>
               </li>
@@ -24,6 +29,7 @@ export default class App extends React.Component<{}> {
 
           <hr />
           <Route exact path="/" component={Home} />
+          <Route path={CRED_EXPLORER_ROUTE} component={CredExplorer} />
           <Route path={ARTIFACT_EDITOR_ROUTE} component={ArtifactEditor} />
         </div>
       </Router>

src/app/credExplorer/App.js

@@ -0,0 +1,26 @@
+// @flow
+
+import React from "react";
+import {StyleSheet, css} from "aphrodite/no-important";
+
+type Props = {};
+type State = {};
+
+export default class App extends React.Component<Props, State> {
+  render() {
+    return (
+      <div>
+        <header className={css(styles.header)}>
+          <h1>Cred Explorer</h1>
+        </header>
+        <p>Welcome to the SourceCred Explorer!</p>
+      </div>
+    );
+  }
+}
+
+const styles = StyleSheet.create({
+  header: {
+    color: "#f0f",
+  },
+});

src/app/credExplorer/App.test.js

@@ -0,0 +1,13 @@
+// @flow
+import React from "react";
+import ReactDOM from "react-dom";
+import App from "./App";
+
+require("../testUtil").configureAphrodite();
+
+// Check that PropTypes check out.
+it("renders without crashing", () => {
+  const div = document.createElement("div");
+  ReactDOM.render(<App />, div);
+  ReactDOM.unmountComponentAtNode(div);
+});