Fix Non-updating certificates

Fixes #806

Right, I think I have this nailed (and have a bonus refactor in for good measure)

The problem was only certificates that were draft or had the old style certificate code `open-data-certificate-questionnaire`. Through refactoring, I worked out a non-destructive way of updating the surveys, which previously was only being applied to draft certificates. This now works for all superseded certificates, so the instructions sent out should work for everyone.