substate khan-exercises
interactive-graph: angle: allow disabling reflex angles

Summary:
Adds a checkbox to allow disabling of reflex (>180 degree) angles,
so that we can test if this is where our issue reports are coming from.

Test Plan:
 * visit test.html
 * uncheck "allow reflex angles"
 * try to make a reflex angle
 * make sure that grading still works after spinning the angle around
   a few times

Reviewers: alex

Reviewed By: alex

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D6189