Kill most of phutil_escape_html()

Summary:
This resolves lots of double escaping.
We changed most of phutil_render_tag(, , $s) to phutil_tag(, , $s) which means that $s is now auto-escaped.
Also pht() auto escapes if it gets PhutilSafeHTML.

Test Plan: None.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2432

Differential Revision: https://secure.phabricator.com/D4889