Background Tasks / Task Broker

Do we want to use background jobs or a task broker? Right now, one place where it could be used is sending of notifications. There might/will be other parts of the system that can be turned asynchronous.

If so, do we want to use some time-based system like django-cron, or a task broker like Celery?

Do we want to make it optional or mandatory (i.e. if unavailable, the tasks will instead be run synchrnously)?