Enforcing IAggregateRoot (#273)

- Enforcing it on the Async and EfRepositories
- Making CatalogBrand, Item, and Type AggregateRoots because they are
just lookup tables

src/ApplicationCore/Interfaces/IAsyncRepository.cs

@@ -4,7 +4,7 @@ using System.Threading.Tasks;
 
 namespace Microsoft.eShopWeb.ApplicationCore.Interfaces
 {
-    public interface IAsyncRepository<T> where T : BaseEntity
+    public interface IAsyncRepository<T> where T : BaseEntity, IAggregateRoot
     {
         Task<T> GetByIdAsync(int id);
         Task<IReadOnlyList<T>> ListAllAsync();