Update to Django 1.6, fixing deprecations, tests, and imports along the way [MJ-818]

This introduces a new batch of deprecation warnings, but resolves all of the old ones.

I was unsure where we wanted to go with this, so I stopped at 1.6. I can go to a point release if we want some security fixes.

This is currently (as of April 30, ~2:50 PM) on staging, if anyone wants to check it out. No functionality has been changed.

To run tests, just run `./manage.py test` - the old testapps management command is now unnecessary, but I left it in place. It still works just as well as it did previously.
You will also need the following in your test settings:
`CELERY_ALWAYS_EAGER = True`
`CELERY_EAGER_PROPAGATES_EXCEPTIONS = True`
`current_app.conf.CELERY_ALWAYS_EAGER = True`
`current_app.conf.CELERY_EAGER_PROPAGATES_EXCEPTIONS = True`

All tests pass.