Don't render HTML for "user answered question" transaction if the rendering target is not HTML

Summary:
Resolves T5817. Continuation of D10231.

This corrects the rendering of the "user answered question" transaction so that it does not incorrectly attempt to render the question handle as HTML in emails if the rendering target is not HTML.

Test Plan: Used bin/mail show-outbound to verify that the email didn't contain escaped HTML when answering a question.

Reviewers: Blessed Reviewers, btrahan, epriestley

Reviewed By: Blessed Reviewers, btrahan, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T5817

Differential Revision: https://secure.phabricator.com/D10319