Use Advanced Python Scheduler to schedule digests (instead of celery.beat)

@gwprice @kevinchugh @e0d 

Reimplements identical functionality using APS https://pypi.python.org/pypi/APScheduler.  So far, this has worked in manual tests against the staging environment (whereas celery.beat was failing running under the same conditions).

Deployment-wise, this is envisioned to run under supervisord with the following command (--help works):
    python manage.py scheduler
BROKER_URL must be set properly in order to correctly invoke the celery tasks - most other configuration settings are needed only by the workers and don't apply here.