Extend the ActivityQueueService to optionally use Celery
We originally didn't use Celery to avoid another infrastructure dependency on DjangoLDP. The ActivityQueueService we built isn't meant to compete with Celery, so for clients who want to have multiple workers to get the best performance, we intended that they could extend it with their own package
This issue is for tracking any changes to the core which will be needed to allow that package to extend it