STUD-945

Previously the file upload dialog was PDF- and textbook-specific. The
changes are adding parameters to the FileUpload model for the file
type, and adding an onSuccess callback to the UploadDialog view. Also
moved upload-specific SASS into its own file.

Previously, for a textbook with only one chapter, we allowed that chapter to
not have a title -- under the assumption that it would be shown as a monolithic
textbook, instead of a chaptered textbook. However, the UI doesn't support that
idea, the code was getting a bit messy, and there was no real benefit to the idea
of not having to specify a chapter title. This commit removes that special case,
and ensures that all textbook chapters must have a name and an asset path.

Created a few RESTful API endpoints, which required creating and assigning
arbitrary IDs to PDF textbooks. Changed the Backbone views to save individual
models, instead of saving a whole collection.

Previously, the code would dynamically add a chapter on render() if the textbook
had no chapters, and would remove all empty chapters when the edit view was closed.
Now, the render method doesn't modify the model at all anymore (yay!) and when the
edit view is closed, remove all empty chapters but be sure to leave at least one
left.

Textbooks now have an empty chapterset by default, instead of one with one empty
chapter. The TextbookEdit view dynamically adds an empty chapter if the chapterset
is empty; the close button removes all empty chapters from the chapterset.

All other file types will trigger a validation error