R1:5ad526942bb6

Convert AphrontPanelView to safe HTML (except children)

Summary: Fixes some double escaping and potential XSS.

Test Plan: Looked at homepage.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2432

Differential Revision: https://secure.phabricator.com/D4917