Update message and notification icons to use fonts

Summary: Cleans up spacing, updates to fonts instead of images. Fixed some mobile issues.

Test Plan:
Test with and without counts on desktop, tablet, mobile. Test layout in FF, Chrome, IE.

{F246745}

{F246746}

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D10948