Added a warning if the current topic is hidden

A hidden topic is won't be displayed in the navigation—but the setting for this is hidden behind the _Navigation_ tab. The warning elevates attention to this, while keeping the setting under _Navigation_, where it makes logical sense.

Author: JeremyCaney

OnTopic.Editor.AspNetCore/Areas/Editor/Views/Editor/Edit.cshtml

@@ -38,6 +38,12 @@
           <strong>Warning:</strong> Not all relationships were successfully loaded from the database. To prevent inadvertent deletion of relationships, any relationships that are removed won't be persisted to the database. You can still add new relationships.
         </div>
       }
+      @if (Model.Topic.IsHidden) {
+        <div class="alert alert-warning" role="alert">
+          <button type="button" class="close" data-dismiss="alert"><span aria-hidden="true">×</span><span class="sr-only">Close</span></button>
+          <strong>Note:</strong> This topic is currently hidden and will not be displayed in the site navigation. Customers can still access it, and it will still show up in search results.
+        </div>
+      }
       @if (!String.IsNullOrWhiteSpace(Model.ContentTypeDescriptor.Description)) {
         <div class="alert alert-primary" role="alert">
           @Model.ContentTypeDescriptor.Description