Use the same date rendering display logic for both tooltips and subheaders

Summary: Ref T11816. This could be a little cleaner, but we currently have two copies of the logic. Get them using the same code. Once that's actually working I can go make the code a little prettier.

Test Plan: Viewed Calendar month view tooltips, saw the same values as subheaders.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11816

Differential Revision: https://secure.phabricator.com/D16880