PDF textbooks in Studio

Not quite done yet, but close enough for code review: only missing some styling from @frrrances.

This branch adds a "Textbooks" tab in Studio, and allows course authors to upload multi-chapter PDF textbooks. The frontend is all done using Backbone.js, and @chrisndodge made some improvements to the backend to better handle large files. We have Jasmine unit tests for the frontend, and Django unit tests for the backend.