Update BlazorStandalone template content to new APIs

src/Microsoft.AspNetCore.Blazor.Templates/content/BlazorStandalone.CSharp/App.cshtml

@@ -2,5 +2,5 @@
     Configuring this stuff here is temporary. Later we'll move the app config
     into Program.cs, and it won't be necessary to specify AppAssembly.
 -->
-<c:Router AppAssembly=@(typeof(Program).Assembly)
-          PagesNamespace=@("BlazorStandalone.CSharp.Pages") />
+<Router AppAssembly=typeof(Program).Assembly
+        PagesNamespace="BlazorStandalone.CSharp.Pages" />

src/Microsoft.AspNetCore.Blazor.Templates/content/BlazorStandalone.CSharp/Pages/_ViewImports.cshtml

@@ -1 +1 @@
-@(Layout<MainLayout>())
+@layout MainLayout

src/Microsoft.AspNetCore.Blazor.Templates/content/BlazorStandalone.CSharp/Shared/MainLayout.cshtml

@@ -1,4 +1,4 @@
-@(Implements<ILayoutComponent>())
+@implements ILayoutComponent
 
 <div class='container-fluid'>
     <div class='row'>

src/Microsoft.AspNetCore.Blazor.Templates/content/BlazorStandalone.CSharp/Shared/NavMenu.cshtml

@@ -13,6 +13,7 @@
         <div class='navbar-collapse collapse'>
             <ul class='nav navbar-nav'>
                 <li>
+                    <!-- The 'c:' prefix and parens are workarounds for a Razor compilation issue and will be removed soon. -->
                     <c:NavLink href=@("/")>
                         <span class='glyphicon glyphicon-home'></span> Home
                     </c:NavLink>