Update `HtmlHelper` to use `IHtmlGenerator`
- make a few more methods available as `internal static` in `DefaultHtmlGenerator`
 - remove `IHtmlGenerator.GenerateOption()`; now `internal static`

nits:
- add `IHtmlGenerator.IdAttributeDotReplacement`
- move `DefaultHtmlGenerator.IdAttributeDotReplacement` after constructor
- move `HtmlHelper.ActionLink()` below static methods
- move newly-`internal` methods together in `DefaultHtmlGenerator`
- correct placement of `DefaultHtmlGenerator.GetValidationAttributes()` comment