Use Interactive2.addMovablePoint for interactive-graph's parabola

Summary:
Avoid using graphie.addMovablePoint so that we get a speed boost later on when
wrapping Interactive2.addMovablePoint's in DOM nodes.

Test Plan:
- Run Perseus.
- Verify that Sinusoid and Parabola mode still work as before.

Reviewers: aria

Reviewed By: aria

Differential Revision: http://phabricator.khanacademy.org/D14202