Add `ModelMetadata.HasNonDefaultEditFormat` property
- add `DataType` property to `CachedDataAnnotationsMetadataAttributes` in support of `ComputeHasNonDefaultEditFormat()`
- provide doc comments and flesh out the implementation of `DisplayFormatString` and `EditFormatString`

nits:
- add comment about _not_ overriding `ComputeIsComplexType()` in `CachedDataAnnotationsModelMetadata`
- restore alphabetic order in `CachedDataAnnotationsMetadataAttributes`
- seal `SimpleDisplayText` in `CachedModelMetadata<TPrototypeCache>`, matching MVC 5.2
Move two recently-added `[HiddenInput]` tests into `CachedDataAnnotationsModelMetadataProviderTest`
Test `DisplayFormatString`, `EditFormatString`, and `HasNonDefaultEditFormat`
- `DisplayFormatString` and `EditFormatString` now based on attributes
- `HasNonDefaultEditFormat` is new
- confirm `DataType` and `ScaffoldColumn` in `CachedDataAnnotationsMetadataAttributes`
Address TODO item with new `HasNonDefaultEditFormat`
- avoid overriding a datetime format if format was already customized
- primarily affects the default Date and Time editor templates because they
  are often used due to `[DataType]` attributes with "default" edit formats
Back `HtmlHelper.Html5DateRenderingMode` with new `ViewContext` property
- provides expected scoped semantics
- helps a bit with #448 but does not provide an app-wide default
Test data/time templates
- focus on affect of `ModelMetadata.HasNonDefaultEditFormat` and
  `IHtmlHelper.Html5DateRenderingMode`
- work through `TemplateRenderer` because individual templates only do
  formatting in a few cases