Return safe HTML from all render()

Summary:
This is pretty brutal and it adds some phutil_safe_html().
But it is a big step in the right direction.

Test Plan: None.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2432

Differential Revision: https://secure.phabricator.com/D4905