Use PHUIRemarkupPreviewView in Phame

Summary: Reuse PHUIMarkupPreviewView in Phame for consistency, less custom code. Also, doesn't work (JS issue).

Test Plan: New Post, Edit Post, Save Post

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D14552