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.

textbook.js is apparently being loaded on every page, so I just made sure that
Backbone Associations was also loaded on every page to be sure that the dependency
was always being met.

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