Deprecate phutil_escape_html()

Summary:
My original idea was to return safe HTML from this function.
But we are down to 20 occurrences in Phabricator and you shouldn't need this function in safe HTML world at all.

Test Plan:
  $ arc lint src/applications/audit/controller/PhabricatorAuditListController.php

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4890