Tweak some icon buttons

Summary: Fixes T12577. Some tweaks last week widened the default buttons, but these didn't get retouched.

Test Plan: Review calendar on desktop and mobile.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T12577

Differential Revision: https://secure.phabricator.com/D17709