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