Remove incorrect override of `ModelMetadata.GetDisplayName()`
- this method delegates to a number of `ModelMetadata` properties
- no use cases for overriding; should not be `virtual`
- `ModelMetadata.DisplayName` override using `DisplayAttribute.GetName()` coming in next commit
Add `ModelMetadata.DisplayName` property
- value comes from `Name` property of `[Display]` attribute
- use new property in `@Html.DisplayName()` and `@Html.Label()`; remove associated TODO comments
Correct `ModelMetadata.IsRequired` to honour overrides
- subclasses would not calculate a value because `ModelMetadata` constructor
  set this property
- meant presence of a `[Required]` attribute in the model was ignored
Add more `ModelMetadata` and HTML helper tests
- add `CachedDataAnnotationsMetadataAttributesTest`
- add `CachedDataAnnotationsModelMetadataTest`
- confirm more default property values in `ModelMetadataTest`
- confirm use of `DisplayName` in `ModelMetadataTest`
- add `HtmlHelperDisplayNameExtensionsTest`
- add `HtmlHelperLabelExtensionsTest`
- add `HtmlHelperNameExtensionsTest`