[discuss-only, no merging] 'Register a Birth' in Smartdown and comparison tools

Rather than exceptions could this be fixed in the [register-a-birth YML](https://github.com/alphagov/smart-answers/blob/master/lib/smart_answer_flows/locales/en/register-a-birth.yml#L326-L343)? 

Having exceptions is useful during dev, but i'd say one of the signs of a fully complete transition is having no exceptions (even if that means changing the original content). It's a but ecumenical in this case, but it seems like a good principle for future transitions.