Store indicators in the database

This changes the /gt/indicators endpoint to be the mechanism for starting a calculation request, rather than the endpoint that returns calculated results. Triggering the calculation causes the results of all the indicators to be POSTed to the Django /api/indicators endpoint for storage. Also added is a Django management command  for triggering the calculation.
I fully agree, but doubt I have the Scala skills to pull it off. I'll add a TODO and will try and get some of Eugene's time to help refactor.