Fix flakey cucumber steps

These steps occasionally time out trying to load the preview page. I
haven't been able to figure out why, but they seem to be alot more
reliable when we explicitly visit the preview path rather than using
the `visit_link_href` helper.
Move confirm_unpublish to workflow controller

This is closer to the action it relates to. Also makes it easier to
re-render the unpublish form when there is an error.
Unpublish failures re-renders unpublish form
Update guidance page to show archive information
Prepend "[Archived]" to archived docs page title

I'm using a helper because not every edition show action wraps the
edition in a presenter. Besides, not all edition types have a
presenter.
EditionUnpublisher builds unpublishing reason

Instead of having the controller build the Unpublishing on the edition
we pass in the details to the service object and let it get everything
to the correct state
Require explanation when archiving
Use document format name to describe document

header_title is set to "Operations in #{location}" for fatality
notices.
Reset unpublishing fields when hidden
Hide/reset explanation when auto-redirecting
Fix styling of archive notice on collections page
Rework form and JS

The primary motiviation for this was to add the "required" asterisk
indicators to the required fields. Because each version of the
unpublish form has different requirements, it no longer made sense to
do everything in Javascript (it would have made the already
complicated show/hide logic in the JS even more so).

The options were to convert the form to a mustache template that could
be re-rendered with the appropriate parts visible, or to render three
separate forms and show the correct one based on the selected
unpublishing reason. I plumped for the later as it felt less
complicated than introducing mustache to admin.

This does mean the template has got a bit hairy, particularly with
the form generation. This is mainly because I'm making sure the form
fields have unique ids to keep our markup valid and to keep stuff
like preview.js happy.
Merge pull request #1057 from alphagov/60500360-archiving-fixes

Archiving tweaks

https://www.pivotaltracker.com/story/show/60500360