WIP: pages: domain: fix DNS records, must end with a '.'

What does this MR do?

This is a minor UI tweak to help make correct DNS records for custom pages domains. In case the domain is a root domain (example.com) a type record CNAME must not be used! I tried to clarify that. Further a DNS record must end with . after the domain otherwise the domain would be appended mypages.example.com CNAME ... would result in mypages.example.com.example.com. CNAME .... I know a few DNS providers don't allow to use the dot . after the domain depending on the interface and validation they use, however, in this case a proper error is shown, while in the case of a missing dot the record will be created in a unexpected way.

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• [-] Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
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