Calculate `ModelMetadata.DataTypeName` based on metadata
- helps MVC helpers like `@Html.EditorFor()` select the correct template
- #933

nit: add XML doc for `DataTypeName` in base `ModelMetadata`
Correct minor problem in `CachedDataAnnotationsModelMetadata.ComputeDisplayFormatString()`
- only affects an extreme corner case: user sets `metadata.EditFormatString` then reads
  `metadata.DisplayFormatString`
- an extreme case because `EditFormatString` is normally set only when
  `DisplayFormatString` is set and, if set, it's to the same value
- happened to see this while updating `CachedDataAnnotationsModelMetadata` for this PR

nit: an -> a in an adjacent XML comment in `CachedDataAnnotationsModelMetadata`