Make assignment asynchronous

[naderm]

Currently the assign_shifts view hangs for about a minute while members are being assigned to shifts. Ideally, this task should be run as a background job as the Workshift Manager doesn't need to interact with it past start it and being notified of its completion.

We currently use django_cron to schedule periodic tasks, but maybe this would be a good time to look at Celery? It seems to be the popular thing for job queues and it supports both scheduled and asynchronous tasks.