Tweaking css to better accomodate month view week numbers on mobile.

Summary: Fixes T8249, Tweaking css to better accomodate month view week numbers on mobile.

Test Plan: Open month view on mobile and observe that week and day numbers are aligned.

Reviewers: chad, epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T8249

Differential Revision: https://secure.phabricator.com/D12910