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.

**Note:** https://github.gds/gds/alphagov-deployment/pull/762 should be merged before this, or drafts will disappear from preview.

/cc @camilleldn