- correct file and class names for some HH extensions
- three class names were correct but didn't match containing file
- three class and file names matched but didn't start with HtmlHelper
- clean up trailing whitespace and long lines in changed Extensions.cs files
- `HtmlHelperPartialAsyncExtensions`, `HtmlHelperRenderPartialAsyncExtensions`
and `HtmlHelperValidationExtensions` lacked some `[NotNull]` attributes
- merge extension files by concept
- Display / DisplayFor / DisplayForModel methods all into `HtmlHelperDisplayExtensions`
- Partial / RenderPartial methods all into `HtmlHelperPartialExtensions`
- use `IHtmlHelper<TModel>` everywhere
dougbu
Ryan Nowak
David Fowler