InteractiveGraph: "unlimited" click-to-add-points

Summary:
Added new option to InteractiveGraph points type. This mode allows users
to create new points by clicking anywhere on the graph, and remove them
by dragging them to the trash can.

Test Plan:
visit /devadmin/content/items/new
Add an InteractiveGraph widget
Choose "unlimited" for the points dropdown
add and remove points
try previewing and make sure that grading works (empty when no points
created, correct when matching, incorrect when not matching)

Reviewers: alex

Reviewed By: alex

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D4648