Adds mechanisms for disabling the debug toolbar

The Django debug toolbar can now be permanently disabled for a dev
server by setting DEBUG_TOOLBAR=False in local_settings.py, or
temporarily disabled with debug_toolbar=f in the request query string.
Fixes persistence of debug toolbar preference

The code for keeping the debug_toolbar GET parameter across page
navigations wasn't working, and probably isn't possible.

Changes the code so that the state is stored in request.session.
Merge pull request #788 from learning-unlimited/debug-toolbar

Debug toolbar