Update pdfjs

* Upgrades pdf.js to the latest stable version (1.0.473) as an attempt to voodoo-program some bugs away.
* Fix printing PDFs. https://www.pivotaltracker.com/n/projects/984568/stories/73355646
* Allow hiding the Print and Save buttons for non-shareable PDFs. https://www.pivotaltracker.com/n/projects/984568/stories/79549978

The hiding stuff is some pretty weaksauce security-through-obscurity bullshit because removing the buttons entirely silently breaks loading the PDF. https://github.com/mozilla/pdf.js/issues/2611