Only send Phame Post body on new creation

Summary: Right now we're attaching the body of every Phame post on each comment, at least restrict it to newly created objects only.

Test Plan: Write a new post, get full email, leave a comment, get less email.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D14459