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
This is probably fine for now, but since the call to storeIndicators sends the results asynchronously, I think this could potentially result in a situation where the job status is updated before all of the indicator results are stored in the database. Ideally we'd we'd wait until all of the POSTs have completed before calling updateIndicatorJob.