Convert AphrontFormInsetView to safe HTML

Summary: Done by searching for AphrontFormInsetView and verifying all method calls.

Test Plan: /repository/edit/1/tracking/

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2432

Differential Revision: https://secure.phabricator.com/D4845