Make tasks asynchronous

The tasks called inside tasks.py's add_new_nonprofit(...) and perform_aggregate_tasks(...) should be asynchronous. There's some commented-out code for this using Celery, but there's a race condition involving SQL Alchemy. E.g., task A will commit a new nonprofit row to the DB, but queries performed afterwards by task B won't see that new nonprofit row.

Start in tasks.py with add_new_nonprofit(...) and perform_aggregate_tasks(...), then inspect the commented-out Celery code and the SQLAlchemy transaction flow. Reproduce the ordering where task B cannot see task A's committed nonprofit row. Done means the task execution is asynchronous without the reported race, with database changes visible to dependent work.

Written by the indexing model from the issue text.