ziafazal/api-make-gradebook-calculation-async:make grade bookgrade calculation asynchronous

@mattdrayer  @chrisndodge moved grade calculation to celery task to run them asynchronously. FEATURE flag to determine if grade recomputation should be sync is not required since in test environments celery is configured to with `CELERY_ALWAYS_EAGER = True` which means celery task would run as local blocking functions so tests would not fail.
Why did we move away from passing object/instance references here in favor of the ids?  I'd prefer if we could keep the signal more general, unless there's been a recommendation to pass specific pieces of data in signals versus references.  We don't necessarily know how this signal will be interpreted by downstream listeners.