Guidance on certs and subdomain names
Remove suggestion of www-preview and www-dev

We don't want to encourage using the same wildcard certificate for
production, staging and development environments.
Fix meta-data and more descriptive name
Explicit advice against bad idea
Add hyphenated options back in

Hyphens in themselves are not a bad thing. We now have explicit advice
against using the same certificates in different environments.
Use root-relative URLs with .html suffix

I don't think this is strictly necessary, but it's what we're doing
elsewhere in the product. Homogeneity is sometimes a good thing.
Merge pull request #257 from alphagov/service-sub-domains

Guidance on certs and subdomain names