Fix calendar display on profile.

Summary:
...maybe anyway because I can't reproduce it live. This diff does two things that should help with bugginess though - uses $viewer rather than $user (...$user is who we are looking at...) *AND* upgrades a Conpherence util class to Calendar, and said util class has unit tests and came about from fixing a similar bug in Conpherence back in the day.

Wrote some comments in the util class because I think it has a tendency to trip people up. These comments are not partciularly good however.

Test Plan: viewed user profile - looked good. viewed conpherence - looked good. ran unit tests - they passed. (note I would also like to push this live and verify Chad's profile is fixed on secure.phabricator.com)

Reviewers: epriestley, chad

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D8341