Add space under comment previews

Summary: Fixes T11166. Adds some class, and space to the preview widget.

Test Plan: Test Maniphest, Ponder, etc, without a footer.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T11166

Differential Revision: https://secure.phabricator.com/D16168