Use phutil_utf8_convert() in Phabricator

Summary: See D3252.

Test Plan: This one is nasty to test, I'm going to make some coffee first.

Reviewers: davidreuss, vrana, btrahan

Reviewed By: davidreuss

CC: aran

Maniphest Tasks: T452

Differential Revision: https://secure.phabricator.com/D3254