Ensure pipelines integrate properly with various Panels applications

I wrote the basic integration for panel_context/page manager quite some time ago, but we still need to do a pass through and make sure that all the various panels applications (panels_node and panelizer being the biggies) are working correctly with the new pipelines system. the potentially trickiest part is making sure that we've correctly identified whether we're doing a frontend or backend render, and use the appropriate pipeline.