[bug 963088] Fix deprecation warnings in manage.py

To verify:

* Verify the dev server works
* Verify you can run the shell `./manage.py shell`
* Run the tests
* Verify any other manage.py comman (`./manage.py cleanup`?)
* Verify any of our cron jobs (`./manage.py cron update_weekly_votes`?)
* Comment with an `R+` below.

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