Add an ability to hook into webapi method view calling.

WebAPIResource.__call__ no longer directly calls the method-based view
(get, create, etc.). Instead, it calls the new call_method_view()
function, which defaults to calling the view. Subclasses can override
this to perform additional checks or pass additional data without having
to decorate all views.

Reviewed at https://reviews.reviewboard.org/r/6086/