Add compatibility items for controller back from MVC 5
Support specifying paths starting with ~/ for layout and partial views
Fixes #821
Add unit tests to controller properties
Fixing a bad merge on Mvc.sln
Add ModelExpression code generation.

- Sealed the ModelExpression.
- We use the stringified version of the ModelExpression type name to detect ModelExpression properties on TagHelpers. This is so the MvcRazorHost can work in tooling and in runtime.
- Created a GeneratedTagHelperAttributeContext to represent the specific stringified versions of the ModelExpression assets.
- Created an MvcTagHelperAttributeValueCodeRenderer to modify rendering of ModelExpression properties.

#1241
Add tests to validate attribute value rendering.

- Exposed internals from Mvc.Razor.Host to Mvc.Razor.Test so it can use the MvcRazorHost override that takes the IFileSystem.
- Added end-to-end code generation tests for TagHelpers with ModelExpression properties.

#1241
Prevent Roslyn from loading prematurely on cold start
Move ITagHelperActivator DI registration.

- Moved the ITagHelperActivator registration in MvcServices to sit with the RazorPage pieces (it's used on the RazorPage).
Reacting to CLR package versioning changes
Add TagHelperSample.Web
Address tag helper sample code review comments and reflect offline discussion
- lots of new comments in Create.cshtml