Make $deg0 use \approx for approximate angles

Summary:
 * Adds the same processing logic used by $len[0-9] to $deg[0-9],
   which displays an approximation symbol before the number if the
   number is rounded.
 * Refactors said logic into KhanUtil.toFixedApprox

Test Plan:
 * Make a movableAngle in the transformer widget in perseus
 * Make a movablePolygon in the interactive-graph widget in perseus

Reviewers: eater

Reviewed By: eater

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D5671