Add Description Header/Icon in Pholio

Summary: Add described, simple header and icon with divider.

Test Plan: Tested on an existing mock and created a new mock with and without a description.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D8181