Add fitbit notification and subscription

All the data update tasks are done in the background by
celery.
Fix notification parse error and celery db setup

The notification data is sent to us as a file, not as form-data.
Commit to db after updating activities.
Add retry for DB errors
Stop using celery to run tasks

Celery seems a bit heavy for this job. Just use gevent to run the
tasks on our own.
Remove celery test task
Remove celery test task import
Add app context for Task
Store update notification in DB
Use datetime as update_time type
Merge branch 'feature/fitbit' into develop