Use standard infrastructure for Feed in Audit

Summary: Ref T4896. Instead of using custom stuff, use standard stuff.

Test Plan: Viewed a bunch of feed stories and published some over the Asana bridge.

Reviewers: joshuaspence, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4896

Differential Revision: https://secure.phabricator.com/D10114