Enforcing IAggregateRoot (#273)

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

src/Infrastructure/Data/EfRepository.cs

@@ -12,7 +12,7 @@ namespace Microsoft.eShopWeb.Infrastructure.Data
     /// https://blogs.msdn.microsoft.com/pfxteam/2012/04/13/should-i-expose-synchronous-wrappers-for-asynchronous-methods/
     /// </summary>
     /// <typeparam name="T"></typeparam>
-    public class EfRepository<T> : IAsyncRepository<T> where T : BaseEntity
+    public class EfRepository<T> : IAsyncRepository<T> where T : BaseEntity, IAggregateRoot
     {
         protected readonly CatalogContext _dbContext;