Update admin UI to match mockup

Use some JS to selectively show/hide form fields, based on the
currently selected reason for unpublishing.
Show appropriate notice when archiving
Tweak frontend CSS for archive notice

Some frontend eyes to check this over would be appreciated.
Prepend "Archived:" to titles when appropriate

This is so it's clear in external search results that these pages
have been marked as archived, as-per the pivotal story.