Use more explicit CSS class names for boxes in configforms.

The configforms code now uses some better CSS class names for boxes. The
title of a box now uses "box-title", and is wrapped in a "box-head"
element. The main part is now "box-main".

This makes these easier to style without conflicting with existing
"main" and "title" classes.
Fix leaking data between TemplateHooks and add sandboxing.

TemplateHooks could easily leak data into the main context when
rendering, overriding variables provided by other contexts. While
TemplateHook tries to push/pop, subclasses can always override and
forget to do so. They can also potentially raise an exception, breaking
the rendering of the page.

To work around all this, we now push/pop in template_hook_point, to
prevent any leakage, and catch exceptions to help sandbox.

Reviewed at https://reviews.reviewboard.org/r/5761/
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/
Merge branch 'release-0.7.x'