Conpherence - add daily date dividers

Summary: nice title. Fixes T3203. If its been N days and now its Tuesday, it just shows a single marker for Tuesday.

Test Plan: Viewed a conpherence and there were date dividers!

Reviewers: epriestley, chad

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T3203

Differential Revision: https://secure.phabricator.com/D6081