Adding chapters causes errors

[slifty]

If you add a chapter to your book the book rendering fails.

The short term solution is to remove the UX for chapters, but the long term solution is to make sure chapter functionality work properly.

[slifty]

The ultimate issue here is that the renderer doesn't have the concept of a chapter -- its API takes a list of URLs (and has no concept of a chapter).

This is quite unfortunate, and trickles back through the python as well (which is where the issue lies -- the python assumes each item has a URL, but as we know, Chapters have no such thing).

As a first step, I'm going to disable chapters from the UX so that nothing is broken in the short term.

After that I believe we would like to keep the concept of chapters. This will mean updating the node to accept chapters in between urls. I think this can be done easily enough -- update the API so that it takes a list of "items" and if the item is a URL it is treated as such, and otherwise it is treated as a chapter.

Open question: how should chapters appear in the table of contents?

[slifty]

This issue is no longer user-facing, but it will stay open for now because it would be good to add support for chapters back.