Cleanup tabs in project.json files
Use nameof when initializing `GeneratedTagHelperContext`

nits:
- missing blank line before a comment
- cleanup line breaks in `RazorPage.CreateTagHelper`
Tag Helpers: add `ModelExpression` class to support `Expression<Func<TModel, TValue>>` attributes
- includes new `RazorPage<TModel>.CreateModelExpression<TValue>()` method
- #1240

nit:
- regenerating the resources reordered Microsoft.AspNet.Mvc.Core's Resources.designer.cs
Add copyright notices to new files
Add missing access modifier
Add ITagHelperActivator.

- The TagHelperActivator enables dependency injection via properties and allows access to the ViewContext.
- This replaces the ICanHasViewContext mechanism that we had in place before.
- Added tests and fixed up existing to work with new format for providing ViewContext.

#1258