Add comments to internal Phame Posts

Summary: Adds commenting to Phame Posts, also testing a new "document comment style". Unsure about it but Phame is a prototype so good place to explore.

Test Plan: Leave some comments, see some comments, test show/hide.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T9746

Differential Revision: https://secure.phabricator.com/D14451