Add a “version select” to the bridged app (#449)

Summary:
The bridge now lets you select any version of the app that you want, as
long as that’s V1, because that’s the only version that exists. We’ll
add a V3 version shortly.

This implements Step 2 of the plan described in #448.

Test Plan:
In `yarn start` and `node bin/sourcecred.js start`, note that navigating
to `/` redirects to `/v1`, and that the cred explorer works.

wchargin-branch: bridge-select

src/bridge/app/index.js

@@ -1,6 +1,12 @@
 // @flow
 import React from "react";
 import ReactDOM from "react-dom";
+import {
+  BrowserRouter as Router,
+  NavLink,
+  Redirect,
+  Route,
+} from "react-router-dom";
 import "./index.css";
 import V1App from "../../v1/app/App";
 import registerServiceWorker from "./registerServiceWorker";
@@ -9,5 +15,20 @@ const root = document.getElementById("root");
 if (root == null) {
   throw new Error("Unable to find root element!");
 }
-ReactDOM.render(<V1App />, root);
+ReactDOM.render(
+  <Router>
+    <React.Fragment>
+      <strong>Select version:</strong>
+      <ul>
+        <li>
+          <NavLink to="/v1">V1</NavLink>
+        </li>
+      </ul>
+      <Route exact path="/" render={() => <Redirect to="/v1" />} />
+      <hr />
+      <Route path="/v1" component={V1App} />
+    </React.Fragment>
+  </Router>,
+  root
+);
 registerServiceWorker();

src/v1/app/App.js

@@ -1,20 +1,20 @@
 // @flow
 
 import React from "react";
-import {BrowserRouter as Router, Route, NavLink} from "react-router-dom";
+import {Route, NavLink, type Match} from "react-router-dom";
 
 import CredExplorer from "./credExplorer/App";
 
-export default class App extends React.Component<{}> {
+export default class App extends React.Component<{match: Match}> {
   render() {
-    const CRED_EXPLORER_ROUTE = "/explorer";
+    const {match} = this.props;
+    const CRED_EXPLORER_ROUTE = match.url + "/explorer";
     return (
-      <Router>
       <div>
         <nav>
           <ul>
             <li>
-                <NavLink to="/">Home</NavLink>
+              <NavLink to={match.url}>Home</NavLink>
             </li>
             <li>
               <NavLink to={CRED_EXPLORER_ROUTE}>Cred Explorer</NavLink>
@@ -23,10 +23,9 @@ export default class App extends React.Component<{}> {
         </nav>
 
         <hr />
-          <Route exact path="/" component={Home} />
+        <Route exact path={match.url} component={Home} />
         <Route path={CRED_EXPLORER_ROUTE} component={CredExplorer} />
       </div>
-      </Router>
     );
   }
 }