Exclude topic and descendants if IsPrivateBranch

Some branches are intended not exposed publicly. This could be because they're intended for internal use, such as `Configuration`. Or it could be because they're exposed to the website, but are not publicly accessibly, such as `Administration`. In these cases, these branches should not be included in the `SitemapController` results.

This was previously supported by adding the `NoIndex` any page or container (#38). This was due to a bug, however, and resulted in publicly available pages being excluded from the `SitemapController` if an ascendant topic was marked as `NoIndex`.

The recursive `NoIndex` is still supported for `Container` content types, but that's a special condition since `Container` is already excluded from the `SitemapController`, and thus its `NoIndex` attribute _must_ apply to its descendents. That's not intuitive behavior, however, and should be considered deprecated.

The `IsPrivateBranch` offers a solution that addresses both of these scenarios. As the name suggests, it is not intended to be used for publicly available pages, and thus won't affect the `NoIndex` treatment on individual pages—and, for that reason, it doesn't need to be added to e.g. the `TopicViewModel`. But it will accomplish what the recursive `NoIndex` was (unintendedly) used for: to exclude otherwise unavailable pages from the `SitemapController` both to avoid crawl errors and, potentially, to maintain data privacy.

This satisfies the needs of #72.

Author: JeremyCaney

OnTopic.AspNetCore.Mvc/Controllers/SitemapController.cs

@@ -198,6 +198,7 @@ private IEnumerable<XElement> AddTopic(Topic topic, bool includeMetadata = false
       | Validate topic
       \-----------------------------------------------------------------------------------------------------------------------*/
       if (
+        topic.Attributes.GetBoolean("IsPrivateBranch") ||
         topic.ContentType.Equals("Container", StringComparison.OrdinalIgnoreCase) &&
         topic.Attributes.GetBoolean("NoIndex")
       ) return topics;