Fixed #13910 -- Added generator based rendering (streaming) of templates and supporting views

- Added stream_to_response shortcut, similar to render_to_response
- Added stream shortcut, similar to render
- Added stream method to Template and Node classes
- Added stream functionality to defaulttags that would benefit from it
- Added stream function to template.loader
- Added StreamingTemplateResponse and supporting generic StreamingTemplateView
-- Rearranged inheritance of generic.base views to support both traditional and streaming views
-- Added StreamingTemplateResponse and SimpleStreamingTemplateResponse to complement the
   exsting non-streaming template response classes