Add angle degree labels to the transformer widget

Summary:
Adds an angle measure to angles in the transformer widget.
Depends on D5671 for approx symbols.

Test Plan:
 * Make a transformer in test.html
 * Add an angle

Reviewers: eater

Reviewed By: eater

CC: alex, alpert

Differential Revision: http://phabricator.khanacademy.org/D5672