React to identity changes

Author: HaoK

samples/MusicStore.Spa/Startup.cs

@@ -2,6 +2,7 @@
 using Microsoft.AspNetCore.Authorization;
 using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Hosting;
+using Microsoft.AspNetCore.Identity;
 using Microsoft.AspNetCore.Identity.EntityFrameworkCore;
 using Microsoft.EntityFrameworkCore;
 using Microsoft.Extensions.Configuration;