Add ITagHelperActivator and concrete implementation.

- 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
why have the property at all?  can use `_tagHelperActivator` directly in the one place it's used