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
Removes the need to do a null check in the method is all, separates the concerns of initializing the object.