Add endpoint for kicking off indicator calculations

The new endpoint creates an IndicatorJob object and
submits a job to an indicators queue for celery.

Celery processes one job a time, with a timeout currently
set to 10 minutes for processing a job, retrying 3 times
possibly.

Celery tasks were split into 2 queues:
  1. datasources - used for processing datasource imports (osm, gtfs, etc)
     because these can be done concurrently without too much overhead
  2. indicators - used for calculating indicators; cannot be done concurrently
     so limited to one celery worker with timeouts and retries set
Update IndicatorJob status once all indicators calculated