Migrations to an app

The main server owns the migrations, it's the server part,
it's simpler.

It's Yavsc, not one of its lib.

src/Yavsc/Controllers/Contracting/DoController.cs

@@ -13,6 +13,7 @@ namespace Yavsc.Controllers
     using System.Threading.Tasks;
     using Yavsc.Helpers;
     using Microsoft.EntityFrameworkCore;
+    using Yavsc.Server.Helpers;
 
     [Authorize]
     public class DoController : Controller