Ability to archive Phame Posts

Summary: Ref T9897. Adds ability to Archive a Phame Post (only visible under ApplicationSearch).

Test Plan: Archive a post, re-publish it, search for it, archive it again. View Home, Blog, Live pages.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T9897

Differential Revision: https://secure.phabricator.com/D16104