Adding 2FA Authenticator Support (#66)

* Adding support for 2fa, more auth options

* WIP getting auth stuff working

* Added Manage views. 2FA working now for MVC app.

* Switching to using a controller for no-UI logout scenario

* Adding Razor Pages impl of 2FA auth stuff. Works.

src/WebRazorPages/Pages/_ViewImports.cshtml

@@ -1,4 +1,6 @@
 @using Microsoft.eShopWeb.RazorPages
 @using Microsoft.eShopWeb.RazorPages.ViewModels
+@using Microsoft.AspNetCore.Identity
+@using Infrastructure.Identity
 @namespace Microsoft.eShopWeb.RazorPages.Pages
 @addTagHelper *, Microsoft.AspNetCore.Mvc.TagHelpers