Fix first new edited message in full Conpherence spacing

Summary: In new Conpherences, add some padding to the conpherence-edited rule if it's the first child of conpherence-messages.

Test Plan: Test a new Conpherence

Reviewers: epriestley, btrahan

Reviewed By: btrahan

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12175