let pht expect PhutilSafeHTMLProducerInterface

Summary: phutil_escape_html can handle these objects, but pht needs to know to call it.

Test Plan: Render a couple of things? I'm not sure how much damage this can do.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D16244