GitLab

This makes it possible to publish a book with something like
/admin/publish/user/en/latest
if "latest" is a descriptor for the version which points to the
"master" branch. Previously, you had to specify "master" in order to
publish. This change makes the publishing process more flexible and
intuitive.

 Rationale: books yaml files are not really configuration for the app,
 they're really the source of data for the app, kind of like a database
 would be. I think moving them up to the top level makes for a more
 intuitive place to find these files. Plus, if we want extensions
 maintainers to be adding their own books to the system, this new
 organization scheme makes the process of adding a book yaml file just a
 tad easier for people who are unfamiliar with the inner workings of
 this app.

I don't think we need this command because running mkdocs
directly is easy enough.

* classes for Library, Book, Language, and Version
* home page using these objects now and working well
* other things still broken in the midst of refactor

If a book has only one language and only one branch then turn the book
title into a link to the book and don't show anything else. Also use
similar logic to present cleaner lists for books that have only one
language and languages that have only one branch.

book sorting

remove the need for a "secret" when publishing via /listen

looks good!

Mark master branch as stable for Dev Guide

specifying that the Dev Guide can use the master branch as a stable version so that it will show up on the docs home page

removing to-do list from Readme (now using issues instead)