Feature/queue indicator calculations

Adds a new endpoint to create 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
Hmm - yea I imagine if we were using that we would still need to store the result on the object anyways to have a persistent record of the indicator job, not sure though. Might be worth looking into if this starts to get more complicated.