Have AphrontView implement PhutilSafeHTMLProducerInterface

Summary: Allows views to work like tags.

Test Plan: Implemented a few completely arbitrary render() / singleView simplifications. I just picked some that were easy to test. I'll do a more thorough pass on this in a followup; these calls don't really hurt anything.

Reviewers: chad, vrana

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D5306