Make icons 2 px taller, 4 px larger.

Summary: Makes the visual size 2px larger and hit area 4px bigger on notification and message icons.

Test Plan: Review icons in sandbox, test new layout with notifications or messages.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5127