Add week numbers to Calendar month view.

Summary: Closes T8184, Add week numbers to Calendar month view.

Test Plan: Calendar month view should now show week numbers, incrementing on Mondays, regardless of week start day.

Reviewers: chad, #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T8184

Differential Revision: https://secure.phabricator.com/D12886