Add `ModelMetadata.HtmlEncode` property
- use new `ModelMetadata.HtmlEncode` property in HTML helpers
 - conditionally in default HTML display and editor object templates (e.g.
   `@Html.DisplayFor()`) when value is non-null and the template is invoked
   with template depth greater than 1
 - unconditionally in `@Html.DisplayText()` and `@Html.DisplayTextFor()`
- test new property and its use

Also test `DisplayTextExtensions`
- unit test gap