Use most recent transaction in Conpherence notification menu

Summary: Not sure this ever worked correctly, but now once we have a supported action, skip the rest of the transactions. Currently you'll see a random old post.

Test Plan: Test multiple rooms in various states with new messages, edits, new room titles, etc.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D16660