Improve support for book structures

by Molly White for Wikimedia

I will improve support for wikis like Wikisource and WikiBooks, whose content is structured in a book format. I will improve the existing BookManager extension to allow these wikis to structure pages and sections of a book into an easily-navigable unit. Users will be able to enter information about the organizational structure of a book into a form, which will store the book's structure. This will then be used to (optionally) auto-generate navigation bars.