Merge branch 'feature/Container-Response' into develop

`Container` topics are meant for internal organization, not external viewing. Currently, accessing one will generally throw a 500 error since there won't be a corresponding view associated with it. That's not the best experience, as it's an _expected_ error. Instead, the `ValidateTopicAttribute` has been updated to throw a 403 Unauthorized error when a `Container` is accessed directly. This is similar to how e.g. `List` (and `List` descendants) are handled today. Unlike `List`, however, children of `Container` can still be viewed.

The complements a recent (but poorly documented) update in which the `SitemapController` was updated to exclude the `Container` from its output—while still including its children (87b6d1b).

As an alternative to throwing a 403 Unauthorized, the `Container` content type may optionally be configured to support a `Url` attribute, thus allowing editors to configure an explicit redirect, if appropriate.

Author: JeremyCaney

OnTopic.AspNetCore.Mvc/ValidateTopicAttribute.cs

@@ -116,6 +116,17 @@ public override void OnActionExecuting(ActionExecutingContext filterContext) {
         return;
       }
 
+      /*------------------------------------------------------------------------------------------------------------------------
+      | Handle containers
+      >-------------------------------------------------------------------------------------------------------------------------
+      | Like nested topics, containers are not expected to be viewed directly; if a user requests a container, return a 403 to
+      | indicate that the request is valid, but forbidden. Unlike nested topics, children of containers are potentially valid.
+      \-----------------------------------------------------------------------------------------------------------------------*/
+      if (currentTopic.ContentType == "Container") {
+        filterContext.Result = new StatusCodeResult(403);
+        return;
+      }
+
       /*------------------------------------------------------------------------------------------------------------------------
       | Handle page groups
       >-------------------------------------------------------------------------------------------------------------------------