Add `ModelMetadata.HtmlEncode` property
- use new `ModelMetadata.HtmlEncode` property in HTML helpers
 - specifically 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
- similar to MVC 5.2 commit [2b12791aee4f](https://aspnetwebstack.codeplex.com/SourceControl/changeset/2b12791aee4ffc56c7928b623bb45ee425813021)

nits:
- remove dupe `null` check in `DefaultDisplayTemplates.ObjectTemplate()`
- move backing fields initialized with constants together in `ModelMetadata`