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 don't know the solution yet, but this is going to end up being pretty long by the time we get all the indicators created; it would be nice to for it to automatically determine the indicators to store so this could be done in a loop. That would also save us from having to remember to add new indicators to this function in order to store them.