Allow HTML versions to be previewed.

Not strictly part of this story, but I was getting fed up of having to
publish an edition just to view the HTML version.
CSS for calls to action on HTML versions
simplify preview_html_version_path helper
Merge pull request #795 from alphagov/49185519-cta-govspeak-on-html-pubs

Adds styling to call to actions on HTML publications

https://www.pivotaltracker.com/story/show/49185519
https://www.pivotaltracker.com/story/show/46571725