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
also copy the name used elsewhere for the same information: `_injectActions`.  the only use of `_activationInfo` occurs in `RazorPageActivator`, where it makes sense because `TValue` is `PageActivationInfo` (a private class).