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