Simplify Fly deploy.py

[ehmatthes]

See conversation in #289.

• [x] Use --json for checking secrets.
• [x] Once available, use --json for fly postgres users list.
• [ ] Consider simpler check for DATABASE_URL in secrets, to see if a database has already been attached. See #297.
• [x] Look at code for getting deployed URL. Probably not worth changing, because we want to call open for automate_all runs anyway.
• [x] Merge.