Add some missing test coverage
Extract external logic from consultations

Ready for sharing and re-use for publications
DRY up consultations#show
Publications can be external

I've made a half-hearted attempt to make frontend look presentable.
Going to handover to someone who knows what they're doing to tidy up
the frontend/CSS.
Move the external publication language
Tidy 'This 'document' is hosted on another website'

• remove paragraph and repitition
• format and style in the same way as external site pages and inline with style guide lines
updating external link for consultations inline with other publication types
Merge branch 'offsite-publications' of github.com:alphagov/whitehall into offsite-publications

Conflicts:
	app/models/publication.rb
	app/views/consultations/_document_summary.html.erb
	app/views/consultations/show.html.erb
	app/views/documents/_attachment_full_width.html.erb
	features/step_definitions/publication_steps.rb
	test/unit/publication_test.rb