R1:6bb7a282b1b5

Convert AphrontFormControl to safe HTML

Summary: Everything here now should properly handle plain strings and safe HTML.

Test Plan: /settings/panel/display/

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2432

Differential Revision: https://secure.phabricator.com/D4826