Month view header should display month name of the month completely shown, not leading or trailing month.

Summary: Fixes T8229, Month view header should display month name of the month completely shown, not leading or trailing month.

Test Plan: Open month view for May. Month view header should be May, not April.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T8229

Differential Revision: https://secure.phabricator.com/D12887