Make pht() return PhutilSafeHTML when passed PhutilSafeHTML?

Summary: See D4645 for discussion. I think the alternative is a ton of `phutil_escape_html(pht(...))`.

Test Plan: Unit tests.

Reviewers: vrana

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D4646