Add a landing page

Summary:
This adds a dummy landing page. We’ll want to actually put nice content
on it. For development convenience, I’m totally fine with having the
`yarn start` launch `/explorer` instead of just `/`.

Test Plan:
Run `yarn start` and note that the navigation works.

wchargin-branch: landing-page

src/app/App.js

@@ -1,19 +1,12 @@
 // @flow
 
 import React from "react";
-import {IndexRoute, Route, Router, browserHistory} from "react-router";
+import {Router, browserHistory} from "react-router";
 
-import Page from "./Page";
-import CredExplorer from "./credExplorer/App";
+import {createRoutes} from "./routes";
 
 export default class App extends React.Component<{}> {
   render() {
-    return (
-      <Router history={browserHistory}>
-        <Route path="/" component={Page}>
-          <IndexRoute component={CredExplorer} />
-        </Route>
-      </Router>
-    );
+    return <Router history={browserHistory} routes={createRoutes()} />;
   }
 }

src/app/HomePage.js

@@ -0,0 +1,9 @@
+// @flow
+
+import React from "react";
+
+export default class HomePage extends React.Component<{||}> {
+  render() {
+    return <h1>Home</h1>;
+  }
+}

src/app/Page.js

@@ -1,11 +1,28 @@
 // @flow
 
 import React, {type Node} from "react";
+import {Link} from "react-router";
+
+import {routeData} from "./routes";
+import * as NullUtil from "../util/null";
 
 export default class Page extends React.Component<{|+children: Node|}> {
   render() {
     return (
       <div>
+        <header>
+          <nav>
+            <ul style={{listStyle: "none", paddingLeft: 0, margin: 0}}>
+              {routeData.map(({navTitle, path}) =>
+                NullUtil.map(navTitle, (navTitle) => (
+                  <li key={path} style={{display: "inline", marginRight: 10}}>
+                    <Link to={path}>{navTitle}</Link>
+                  </li>
+                ))
+              )}
+            </ul>
+          </nav>
+        </header>
         <main>{this.props.children}</main>
       </div>
     );

src/app/credExplorer/App.js

@@ -16,7 +16,7 @@ import type {PagerankNodeDecomposition} from "../../core/attribution/pagerankNod
 
 import * as NullUtil from "../../util/null";
 
-type Props = {};
+type Props = {||};
 type State = {
   repoOwner: string,
   repoName: string,

src/app/routes.js

@@ -0,0 +1,42 @@
+// @flow
+
+import React from "react";
+import {IndexRoute, Route} from "react-router";
+
+import Page from "./Page";
+
+type RouteDatum = {|
+  +path: string,
+  +component: React$ComponentType<{||}>,
+  +title: string,
+  +navTitle: ?string,
+|};
+
+export const routeData: $ReadOnlyArray<RouteDatum> = [
+  {
+    path: "/",
+    component: require("./HomePage").default,
+    title: "SourceCred",
+    navTitle: "Home",
+  },
+  {
+    path: "/explorer",
+    component: require("./credExplorer/App").default,
+    title: "SourceCred explorer",
+    navTitle: "Explorer",
+  },
+];
+
+export function createRoutes() {
+  return (
+    <Route path="/" component={Page}>
+      {routeData.map(({path, component}) => {
+        if (path === "/") {
+          return <IndexRoute key={path} component={component} />;
+        } else {
+          return <Route key={path} path={path} component={component} />;
+        }
+      })}
+    </Route>
+  );
+}