Week day names in Calendar month view should be full names on desktop and short names on device

Summary: Ref T8186, Week day names in Calendar month view should be full names on desktop and short names on device

Test Plan: Open month view, desktop view should display Monday, Tuesday, etc. Shrink browser, days should be Mon, Tue, etc.

Reviewers: chad, epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T8186

Differential Revision: https://secure.phabricator.com/D12859