Disable drafts by default in RAILS_ENV=production

To reduce the chance of a misconfiguration revealing draft content in
production, this changes the default config to not show drafts unless
explicitly overridden.

This removes the `show_transitions: false` option because that's already
the default value for it.
Merge pull request #1221 from alphagov/tweak_flow_preview_config

Disable drafts by default in RAILS_ENV=production