Allow specifying depth of `ITopicRepository.Load()`

[JeremyCaney]

Replace the isRecursive: bool parameter on ITopicRepository.Load() with depth: int, because hierarchical navigation (e.g., rendering a navigation menu from a root several tiers deep) needs precise control over how much of the tree to pull. A full recursive load of the root would force the whole graph to load, defeating the purpose of lazy loading, so the goal is load exactly the region of the graph needed.

Tasks

• C# API (depth replaces isRecursive across Load() and IsLoaded() stub modeling, tests)
• SQL @Depth parameter on LoadTopics stored procedure w/ recursive CTE
  • This replaces existing @LoadDescendants and @LoadChildren parameters
• Allow CachedTopicRepository's existing "sufficiency gate" (i.e., ConvergeLoadState() to be depth-aware
• Sequentialize the HierarchicalTopicMappingService child crawl
  • This fixes the nondeterministic menu ordering and unsynchronized modification of the graph
• Make IsVisible()'s IsHidden and IsDisabled checks non-autoloading
  • These currently trigger unnecessary loading of ExtendedAttributes and references tree traversal