Migrate from South to built-in migrations

[adehnert]

Starting in Django 1.7, Django has a built in migration system similar to South. South does not support Django 1.7 at all. Thus, we'll need to upgrade to the new migrations -- ​https://docs.djangoproject.com/en/dev/topics/migrations/#upgrading-from-south.

I believe that we may not be able to follow the stock instructions, since we've got some data migrations that I think will need extra attention.