- XML comments for changed `TemplateInfo` properties
- correct `DefaultDisplayTemplateTests` and `DefaultEditorTemplateTests` namespaces
- add a couple of low-level `TemplateInfo` tests
- add more `DefaultTemplatesUtilities.GetHtmlHelper()` overloads, all
returning a strongly-typed helper
- also correct that method to ensure template helpers each get a unique
`IHtmlHelper` instance