- #704 part 2 of 2
- change `@Html.Id()` to sanitize return value; was identical to `@Html.Name()`
Copied `TagBuilder.CreateSanitizedId()` and `TagBuilder.Html401IdUtil` from MVC 5.2
- except this `CreateSanitizedId()` returns a valid identifier if first `char` is not a letter
- e.g. "[0].Name"
nits:
- expand variable names, use lots of `var`, put `public` members first
- add doc comments for `CreateSanitizedId()`
Note users will be able to apply different sanitization once we fix#1188.
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