Wire `Assets` into React components (#675)

Summary:
This commit takes the next step toward #643 by exposing `Assets` to our
React components at top level. Components will be expected to pass them
down as appropriate; this commit does not add any actual uses.

Test Plan:
Apply the following patch:

```diff
diff --git a/src/app/Page.js b/src/app/Page.js
index 24c2602..7ac2641 100644
--- a/src/app/Page.js
+++ b/src/app/Page.js
@@ -24,6 +24,10 @@ export default class Page extends React.Component<{|
                 <Link to="/" className={css(style.navLink, style.navLinkTitle)}>
                   SourceCred
                 </Link>
+                <img
+                  alt="fav"
+                  src={this.props.assets.resolve("/favicon.png")}
+                />
               </li>
               {routeData.map(({navTitle, path}) =>
                 NullUtil.map(navTitle, (navTitle) => (
```

Then, observe that the favicon loads correctly and updates across page
loads and refreshes in the following situations:
  - under `yarn start`;
  - after building the static site and serving from root;
  - after building the static site and serving from another gateway.

wchargin-branch: use-withAssets

src/app/HomePage.js

@@ -4,7 +4,9 @@ import {StyleSheet, css} from "aphrodite/no-important";
 import React, {type Node} from "react";
 import {Link} from "react-router";
 
-export default class HomePage extends React.Component<{||}> {
+import type {Assets} from "./assets";
+
+export default class HomePage extends React.Component<{|+assets: Assets|}> {
   render() {
     const urls = {
       numpyFunding:

src/app/Page.js

@@ -3,12 +3,17 @@
 import React, {type Node} from "react";
 import {Link} from "react-router";
 import {StyleSheet, css} from "aphrodite/no-important";
+
+import type {Assets} from "./assets";
 import GithubLogo from "./GithubLogo";
 import TwitterLogo from "./TwitterLogo";
 import {routeData} from "./routeData";
 import * as NullUtil from "../util/null";
 
-export default class Page extends React.Component<{|+children: Node|}> {
+export default class Page extends React.Component<{|
+  +assets: Assets,
+  +children: Node,
+|}> {
   render() {
     return (
       <div>

src/app/createRoutes.js

@@ -5,22 +5,28 @@ import {IndexRoute, Route} from "react-router";
 
 import Page from "./Page";
 import ExternalRedirect from "./ExternalRedirect";
+import withAssets from "./withAssets";
 import {routeData} from "./routeData";
 
 export function createRoutes() {
   return (
-    <Route path="/" component={Page}>
+    <Route path="/" component={withAssets(Page)}>
       {routeData.map(({path, contents}) => {
         switch (contents.type) {
           case "PAGE":
             if (path === "/") {
-              return <IndexRoute key={path} component={contents.component()} />;
+              return (
+                <IndexRoute
+                  key={path}
+                  component={withAssets(contents.component())}
+                />
+              );
             } else {
               return (
                 <Route
                   key={path}
                   path={path}
-                  component={contents.component()}
+                  component={withAssets(contents.component())}
                 />
               );
             }

src/app/credExplorer/App.js

@@ -2,6 +2,7 @@
 
 import React from "react";
 
+import type {Assets} from "../assets";
 import type {LocalStore} from "../localStore";
 import CheckedLocalStore from "../checkedLocalStore";
 import BrowserLocalStore from "../browserLocalStore";
@@ -17,7 +18,7 @@ import {
   initialState,
 } from "./state";
 
-export default class AppPage extends React.Component<{||}> {
+export default class AppPage extends React.Component<{|+assets: Assets|}> {
   static _LOCAL_STORE = new CheckedLocalStore(
     new BrowserLocalStore({
       version: "2",
@@ -27,11 +28,11 @@ export default class AppPage extends React.Component<{||}> {
 
   render() {
     const App = createApp(createStateTransitionMachine);
-    return <App localStore={AppPage._LOCAL_STORE} />;
+    return <App assets={this.props.assets} localStore={AppPage._LOCAL_STORE} />;
   }
 }
 
-type Props = {|+localStore: LocalStore|};
+type Props = {|+assets: Assets, +localStore: LocalStore|};
 type State = {|
   appState: AppState,
 |};

src/app/credExplorer/App.test.js

@@ -5,6 +5,7 @@ import {shallow} from "enzyme";
 
 import {Graph} from "../../core/graph";
 import {makeRepo} from "../../core/repo";
+import {Assets} from "../assets";
 import testLocalStore from "../testLocalStore";
 import {DynamicAdapterSet, StaticAdapterSet} from "../adapters/adapterSet";
 
@@ -35,7 +36,9 @@ describe("app/credExplorer/App", () => {
       };
     }
     const App = createApp(createMockSTM);
-    const el = shallow(<App localStore={localStore} />);
+    const el = shallow(
+      <App assets={new Assets(null)} localStore={localStore} />
+    );
     if (setState == null || getState == null) {
       throw new Error("Initialization problems");
     }

src/app/routeData.js

@@ -7,11 +7,18 @@
 // inline syntax.
 
 /*::
+import type {Assets} from "./assets";
 type RouteDatum = {|
   +path: string,
   +contents:
-    | {|+type: "PAGE", +component: () => React$ComponentType<{||}>|}
-    | {|+type: "EXTERNAL_REDIRECT", +redirectTo: string|},
+    | {|
+        +type: "PAGE",
+        +component: () => React$ComponentType<{|+assets: Assets|}>,
+      |}
+    | {|
+        +type: "EXTERNAL_REDIRECT",
+        +redirectTo: string,
+      |},
   +title: string,
   +navTitle: ?string,
 |};

src/app/server.js

@@ -72,7 +72,7 @@ export default function render(
 
   function renderRedirect(redirectTo: string) {
     const component = (
-      <Page>
+      <Page assets={assets}>
         <ExternalRedirect redirectTo={redirectTo} />
       </Page>
     );