Fix XSS in Ponder

Summary: See rP883829e6676fc3412b83b6ab16f7bf5b56b174b8

Test Plan: Verified no XSS with a title like `<b>!</b>`.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D4534