Add MIXPANEL_DISABLE setting.

I ended up only adding the check to the `run()` method, to simplify the implementation, and because I realized that in most cases where you'd set `MIXPANEL_DISABLE` to `True` (i.e. tests) you'll probably also be running with eager Celery tasks, in which case there won't be a task queued anyway.

I also didn't change the name of the `MIXPANEL_TEST_ONLY` setting in this PR; I figured that's a separate change.