Depend on `HtmlString` from `Microsoft.AspNetCore.Html`.

[NTaylorMullen]

HtmlString today inherits from HtmlEncodedString and only adds the Empty property. Aside from back compat it seems odd to have both available; might be nice to have just one right way of saying "this is already encoded".

/cc @Eilon

[Muchiachio]

👍 for leaving only HtmlString and moving it to abstractions.
Most of the time IHtmlString interface is used anyway, which is self explanatory and widely known.

[dougbu]

@NTaylorMullen were you really proposing removing HtmlString? That's the one that's broadly known, in a commonly-referenced namespace, and what users should use. HtmlEncodedString exists primarily because some low-level classes had to be aware of the concept. And back-compat is a great reason to keep the 10 line subclass.

@Muchiachio IHtmlString does not exist in ASP.NET Core MVC.

[NTaylorMullen]

@dougbu Yes, removing HtmlString.

[Muchiachio]

oh sorry I meant IHtmlContent :)

[NTaylorMullen]

Talked a bit offline. This issue should be "remove HtmlString" and i'll file a new issue to rename HtmlEncodedString => HtmlString