HTML versions of consultations

This moves a number of the publication specific functionality out into
modules so that it can also be used by consultations.

Not entirely happy with the `send` in `attachments_presenter.rb` - happy
to field alternatives. Cannot use polymorphic paths as the route names
and the paths aren't quite standard.
Merge pull request #515 from alphagov/html-versions-of-consultations

HTML versions of consultations

https://www.pivotaltracker.com/story/show/48888203