Make django-debug-toolbar >= 1.0 work properly with syncdb and admin.

@wedaly, @jrbl, @stephensanchez: The autodiscovery used by admin will interact badly with Django Debug Toolbar's own auto-setup. So we have to be more explicit about everything and add some settings manually. See:

http://django-debug-toolbar.readthedocs.org/en/1.0/installation.html#explicit-setup

@dianakhuang, @flowerhack, @nedbat: I originally tried the fix used in https://github.com/edx/xblock-sdk/pull/2, but that didn't work for me. Don't know if this might be useful in xblock-sdk as well.