Avoid running Markdown more than you have to

Summary:
Answer area renderer does not work. Someone maybe should commandeer this and make it work. :)

Fixes T2907.

Test Plan: Dragged the point around.

Reviewers: charlie, jack

Reviewed By: jack

Subscribers: jessie, kevin, michelle, eater

Maniphest Tasks: T2907

Differential Revision: http://phabricator.khanacademy.org/D11430