Convert phutil_render_tag() to phutil_tag()

Summary:
So that I could remove it from libphutil.

I'm not sure how much of this code will be reused but it wasn't that much work so that we couldn't flush it away.

Test Plan: Generated Phabricator docs, didn't see any double escaping.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D6503