Improve `HtmlHelper` extension points
- address all of #659 and a bit of #874 (avoid `public virtual` methods in
  `HtmlHelper`)

- make `MetadataProvider` and `GetClientValidationRules()` `public` and
  therefore available to extension methods
- remove unused `GetValidationAttributes()` overload
- make remaining `GetValidationAttributes()` overload (and not
  `GetClientValidationRules()`) `virtual`, allowing derived classes to
  change the attributes without overriding all callers
- reverse `GetValidationAttributes()` and `GetClientValidationRules()`
  parameter order to match precedence
- add `GenerateName()` and `GenerateValidationSummary()` to make
  `protected virtual` method names consistent
- `Name()`, `ValidationSummary()` and `TextArea()` are no longer `virtual`
  because `protected virtual Generate*()` methods exist for all
Make longest `@Html.DisplayForModel()` overload an extension method
- consistent with @Html.EditorForModel()` overloads