Convert all calls to 'IconFont' to just 'Icon'

Summary: Mostly for consistency, we're not using other forms of icons and this makes all classes that use an icon call it in the same way.

Test Plan: tested uiexamples, lots of other random pages.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D15125