Support building media with parallel-installed Django

On systems such as Fedora where older versions of Django are installed as
alternative versions (and not directly importable), we need to ensure that we
are importing exactly the right version of the Django modules.

This patch adds a new variable in the djblets __init__.py that is imported by
both setup.py and build-media.py to ensure that they are both relying on the
same version of Django.

This doesn't get handled automatically by setuptools because this script is
invoked directly via sys.executable() in BuildMedia().

Testing Done:
Successfully built Djblets for Fedora

Reviewed at https://reviews.reviewboard.org/r/5689/