Ensures all back office controllers have the thread culture set, we do this with a new application model targeting all back office controllers.

src/Umbraco.Web.Common/Extensions/UmbracoWebServiceCollectionExtensions.cs

@@ -36,6 +36,7 @@ namespace Umbraco.Extensions
         {
             services.ConfigureOptions<UmbracoMvcConfigureOptions>();
             services.TryAddEnumerable(ServiceDescriptor.Transient<IApplicationModelProvider, UmbracoApiBehaviorApplicationModelProvider>());
+            services.TryAddEnumerable(ServiceDescriptor.Transient<IApplicationModelProvider, BackOfficeApplicationModelProvider>());
 
             // TODO: We need to avoid this, surely there's a way? See ContainerTests.BuildServiceProvider_Before_Host_Is_Configured
             var serviceProvider = services.BuildServiceProvider();