Refactor PHUICalendarMonthView to be a little more readable

Summary: Refactor PHUICalendarMonthView to be a little more readable

Test Plan: Make sure month view still works

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12831