Make qa_loader.py nicer for load and workstation testing

This version:
- generates legible names, messages, and replies
- generates recent dates
- generates source keys
- permits specification of exact numbers:
  - of journalists and sources
  - number of submissions and replies per source
  - fraction of sources starred
  - fraction of sources replied to

The resulting database produces sensible API results, so functions
better for testing the SecureDrop Workstation.

Author: rmol