Update to use new `IHierarchicalTopicMappingService<>`

[JeremyCaney]

OnTopic 3.8.0 makes a breaking change to the LayoutController by replacing the ITopicMappingService dependency with a new IHierarchicalTopicMappingService<> dependency. This centralizes a lot of the reusable logic for various LayoutController implementations while, importantly, also handing over control over caching to the composition root, thus allowing a single LayoutController to be used independent of whether or not the results need to be cached.

Tasks

• [x] Update LayoutController to use IHierarchicalTopicMappingService<>
• [x] Update GoldSimControllerFactory to construct proper dependency graph
• [x] Update TopicViewModel to auto-initialize Children so it can't be assigned to

[JeremyCaney]

This update has been completed as of #(dee9b95).