Import historical data.

@brad So here is what I have for importing historical data. You can see in the view code where it should be imported, but having that call there was breaking tests. (Looks like a Celery issue.)

I tested everything manually with your Misfit credentials, and it's looking good. No unit tests because mocking all of those api calls looks kind of painful, and I ran out of time to do it today.
@brad Yeah, and I think hitting a rate limit error there is pretty likely, so I will put some error handling and recovery there before we merge. Good catch.