Switch PDF Textbooks to use portable URLs

PDF textbooks were using c4x URLs in studio.  This required special remapping code on import and caused issues for courses imported without using the contentstore.  Additionally, the static url remapping in staticbook wasn't using all the arguments that are needed so that static_asset_path handling works properly.

This fix changes the behavior to use portable URLs in studio, adapts the specialized remapping import code to convert textbook c4x URLs to portable URLs to help correct the legacy problem, and corrected the staticbook url mapping call.

@sarina this is another cross cutting PR so I'm not sure who would be best, and thank you as always.