Kill off PublicationType::Consultation

I see no reason for this type to exist anymore - no Publication should
have this type, and Consultations are identifiable without it. This
required some slight tweaking to the MySQL front-end filterer.

(NOTE: the MySQL filterer isn't smart enough to distinguish between
open and closed consultations. That was the case before these changes)
Data fix for publications with legacy type
Now we have removed the lagacy PublicationType::Consultation, we just
need to clean up those half a dozen Publications that have incorretly
been assigned it. We force their type to Guidance, which is what the
type of their latest editions is.
Merge pull request #788 from alphagov/remove-consultation-publication-type

Remove consultation publication type