Ability to archive items

An archived item doesn't show up in search results, but there should be a way of revealing it when browsing the project it belongs to.

Everyone who currently has access keeps it (including public accessibility). There should be an option to direct users to a newer version of the question.

Start by locating the existing search-results and project-browsing entry points for items, then trace how archived items and question versions are represented. Done means archived items can be revealed while browsing their project, existing access is preserved, and users can be directed to a newer question version.

Written by the indexing model from the issue text.