Update templates for Yarn compatibility

templates/KnockoutSpa/ClientApp/components/app-root/app-root.ts

@@ -1,4 +1,5 @@
 import * as ko from 'knockout';
+import * as History from 'history';
 import { Route, Router } from '../../router';
 import navMenu from '../nav-menu/nav-menu';
 

templates/KnockoutSpa/ClientApp/router.ts

@@ -1,5 +1,6 @@
 import * as ko from 'knockout';
 import * as $ from 'jquery';
+import * as History from 'history';
 import crossroads = require('crossroads');
 
 // This module configures crossroads.js, a routing library. If you prefer, you

templates/KnockoutSpa/template_gitignore

@@ -33,6 +33,8 @@ Obj/
 # Workaround for https://github.com/aspnet/JavaScriptServices/issues/235
 !/wwwroot/dist/_placeholder.txt
 
+/yarn.lock
+
 # MSTest test Results
 [Tt]est[Rr]esult*/
 [Bb]uild[Ll]og.*