PDF support

Solves  https://github.com/learningequality/ka-lite/issues/2385 and https://github.com/learningequality/ka-lite/issues/2386. 


Changes:
- add pdfjs library from Mozilla and show it for nodes with the "Document" kind.
- add points and progress tracking when viewing a PDF.
- on content import: Make sure we verify nodes before saving them.
- Start abstracting the common logic of the different content viewers into a common Backbone view.