feat(react): Add '/oauth' to React app

Because:
* We are converting Backbone flows to React

This commit:
* Displays email-first Index page if path is /oauth, for React

closes FXA-6516

Author: LZoog

packages/fxa-content-server/app/scripts/lib/router.js

@@ -288,7 +288,9 @@ Router = Router.extend({
         contentRedirect: true,
       });
     },
-    'oauth(/)': createViewHandler(IndexView),
+    'oauth(/)': function () {
+      this.createReactOrBackboneViewHandler('oauth', IndexView);
+    },
     'oauth/force_auth(/)': function () {
       this.createReactOrBackboneViewHandler('oauth/force_auth', ForceAuthView, {
         ...Url.searchParams(this.window.location.search),

packages/fxa-content-server/server/lib/routes/react-app/index.js

@@ -25,7 +25,7 @@ const getReactRouteGroups = (showReactApp, reactRoute) => {
   return {
     emailFirstRoutes: {
       featureFlagOn: showReactApp.emailFirstRoutes,
-      routes: reactRoute.getRoutes(['/']),
+      routes: reactRoute.getRoutes(['/', 'oauth']),
       fullProdRollout: false,
     },
     simpleRoutes: {

packages/fxa-settings/src/components/App/index.tsx

@@ -324,6 +324,7 @@ const AuthAndAccountSetupRoutes = ({
     <Router>
       {/* Index */}
       <IndexContainer path="/" {...{ integration, serviceName }} />
+      <IndexContainer path="/oauth" {...{ integration, serviceName }} />
 
       {/* Legal */}
       <Legal path="/legal/*" />