The docs building now uses the extensions found in beanbag-docutils,
reducing the amount of code we have to keep maintained here, and
ensuring that they work the same way across all our projects.

It also uses a mirror of the Django 1.6 docs, since Django removed them
from the official website.

The Djblets docs location has also been fixed to use the "dev" docs for
Djblets, rather than the 0.10 docs, for now.

Successfully built the docs.