Add `[HiddenInput]` and `ModelMetadata.HideSurroundingHtml`

Issue #843 
- address related TODO items
- add missing validation messages to `@Html.Editor()` (TODO that wasn't tracked here)
- sort `ModelMetadata.Properties` as MVC 5.2 does (part of #964)

Will resolve the `ReadOnlyCollection` question tomorrow.  Filed #1012 about whether an additional sort key would be worthwhile / justified.