Update Conpherence full message column UI

Summary:
Minor things

• Use radiused avatars like timeline
• Fix edge case with conpherence edited and date markers

Test Plan: Review a number of odd states in Conpherence full.

Reviewers: epriestley, btrahan

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12251