Allow basic html version entry for publications

Currently the HTML version is stored as a seperate model in the
database, and recorded from the publication.
Copy HTML version over on republish
Wrap attachment display logic in a presenter
HTML 'attachment' in presenter when HTML version

We add a virtual 'attachment' to our list when viewing a publication
with an html attachment. This isn't a real attachment, but a PORO which
simulates the fields of an attachment for the purposes of the view.
Basic display of HTML publications
Rename mistakenly named method to show intent
Ensure title/body are protected from XSS
Merge pull request #333 from alphagov/html-versions

First pass at HTML versions of attachments

https://www.pivotaltracker.com/story/show/45760867