When polygon side lengths aren't exact, display an \approx symbol

Summary: T1947

Test Plan: In perseus local mode, with this change in khan-exercises, add a polygon widget that snaps to grid. Note that only when the side length is not exactly the value shown, the \approx symbol is shown.

Reviewers: alex

Reviewed By: alex

Differential Revision: http://phabricator.khanacademy.org/D5136