Introduce DNS advice for new and existing domains

This commit adds some DNS help information when adding domains
or on an ad-hoc basis.  By looking at the information for an app,
the current SSL add-ons and the status of the app we can give useful
and accurate DNS information.

This has already been tested as a concept here:  https://github.com/neilmiddleton/heroku-domains-extended

Note: this does introduce dependency on the app `dominion-dns`, but this is under Heroku control.