Django-fitbit

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- More consistently refresh tokens when needed
- Added refresh_tokens command to manually refresh user fitbit tokens

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- Add exponential back-off and random jitter to task retries
- Enable some configuration around subscriptions:
- FITAPP_SUBSCRIPTIONS: List exactly which subscriptions to retrieve and the order to retrieve them in
- FITAPP_HISTORICAL_INIT_DELAY: The initial delay (in seconds) to wait before retrieving any historic data
- FITAPP_BETWEEN_DELAY: The delay (in seconds) to wait between retrieving each type of resource

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- Add docstrings to all models, help_text to all fields

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- More refresh token bugfixes

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- Refresh token bugfixes
- Use fitbit==0.2.2

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- Integrate with python-fitbit OAuth2 (fitbit==0.2)
- Update documentation to state that `FITAPP_CONSUMER_KEY` should be the OAuth 2.0 client id