Interactive2.MovableLine

Summary: It's a line! that's interactive! 2!

Test Plan:
 * check moving, adding, removing, and grading of:
 * line segments, linear equation, linear system

Reviewers: justinjaffray

Reviewed By: justinjaffray

CC: brianmerlob, alex, alpert

Differential Revision: http://phabricator.khanacademy.org/D7066