- add `CachedDataAnnotationsMetadataAttributes.UIHint`
- set `ModelMetadata.TemplateHint` using `UIHintAttribute` or `HiddenInputAttribute`
- add doc comments for `TemplateHint`-related properties and methods
- add unit tests and use these attributes in functional tests
nits:
- cache and seal `CachedModelMetadata.IsCollectionType`
- correct doc comments for `ModelMetadata.RealModelType`
- add doc comments for `IsCollectionType`-related properties and methods
- add doc comments for `IsComplexType`-related properties and methods
- move `CachedModelMetadata.IsComplexType` right below `IsCollectionType`
- same for related fields and methods