Call `IHtmlHelper` methods from extension methods
- don't call other extension methods
- #847 line 7
- remove extension methods that do nothing but pass through to the interface method
 - weren't exactly ambiguous (interface method wins) but were useless
Prefer `IHtmlHelper` methods elsewhere in our code
- #847 line 7 sub-bullets
- don't call extension methods from our templates
 - partially addressed in PR #934 (for methods changed there)
- don't reference extension methods from XML comments
Correct extension class names to match containing files
- #874 line 5
- `EditorExtensions` -> `HtmlHelperEditorExtensions`
- `SelectExtensions` -> `HtmlHelperSelectExtensions`
Remove incorrect references to `HtmlHelper` class
- extension methods extend `IHtmlHelper`