interactive-graph: click-to-add polygon mode

Summary:
Adds a new "unlimited sides" option to the polygon tool in
interactive-graph, which draws polygons with unlimited points

Test Plan:
 * Make a polygon
 * Set the points type to unlimited
 * Make a shape in the grading area
 * reproduce the shape in the answer area
 * score the question and make sure it is correct if they match
 * change it so the shapes don't match, and score
 * change the shape by deleting random points and adding new ones,
 * make sure score keeps working
 * try making the shape but unclosed, and make sure it grades as
   empty (and not correct), and that other unclosed shapes grade
   as empty (not wrong)

Reviewers: alex

Reviewed By: alex

CC: brianmerlob, alpert

Differential Revision: http://phabricator.khanacademy.org/D6578