This makes it possible to refer to multiple book from a single app.
This also means that Open edX branches will be able to pass bok-choy
tests (at least if the books have already been built.)
TNL-4906 Subsections now use radio buttons, to allow for "hide after due" as a
visibility option. Also, all tabs have been consolidated to "Basic" and
"Advanced", and visibility options have moved there.
Documentation links are updated to assist course authors with the new
visibility options. Tests have also been updated, and the changes suggested
in TNL-4951 are included.
