Create Interactive2.addMovablePolygon and use it in interactive-graph.

Summary:
Before we can wrap polygon's in their own DOM nodes, we'll need them to use the
interactive2.js movables (for interactive-graph in particular). This requires:

- Creating a new MovablePolygon using interactive2.
- Convert the polygon's in interactive-graph to use interactive2 MovablePoints for their vertices.

This diff covers both of those requirements. All of the previous behavior should be preserved, as-is
specified in the test plan.

Depends on D14170.

Test Plan:
- Run Perseus.
- Create an [interactive-graph](http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22%5B%5B%E2%98%83%20interactive-graph%202%5D%5D%22%2C%22images%22%3A%7B%22https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2Fda8df81c78b22f5c69d477d8eabfb583968eaf84.png%22%3A%7B%22width%22%3A400%2C%22height%22%3A70%7D%2C%22https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2Fb59fc02ca1aae800977b8793ed22f647a1aa75ee.png%22%3A%7B%22width%22%3A425%2C%22height%22%3A150%7D%7D%2C%22widgets%22%3A%7B%22interactive-graph%202%22%3A%7B%22type%22%3A%22interactive-graph%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22step%22%3A%5B2%2C2%5D%2C%22backgroundImage%22%3A%7B%22url%22%3Anull%2C%22scale%22%3A1%2C%22bottom%22%3A0%2C%22left%22%3A0%7D%2C%22markings%22%3A%22graph%22%2C%22labels%22%3A%5B%22x%22%2C%22y%22%5D%2C%22showProtractor%22%3Afalse%2C%22showRuler%22%3Afalse%2C%22rulerLabel%22%3A%22%22%2C%22rulerTicks%22%3A10%2C%22range%22%3A%5B%5B-20%2C20%5D%2C%5B-20%2C20%5D%5D%2C%22graph%22%3A%7B%22type%22%3A%22polygon%22%7D%2C%22correct%22%3A%7B%22type%22%3A%22polygon%22%7D%7D%2C%22version%22%3A%7B%22major%22%3A0%2C%22minor%22%3A0%7D%7D%7D%7D%2C%22answerArea%22%3A%7B%22type%22%3A%22multiple%22%2C%22options%22%3A%7B%22content%22%3A%22%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%22calculator%22%3Afalse%7D%2C%22itemDataVersion%22%3A%7B%22major%22%3A0%2C%22minor%22%3A1%7D%2C%22hints%22%3A%5B%5D%7D) with type 'polygon' and three vertices.
    - Verify that the interior highlights when you mouseover the triangle.
    - Verify that clicking and dragging on the interior of the triangle translates it.
    - Verify that none of the vertices can be moved outside of the graph's boundaries.
    - Verify that mousing over a vertex causes it to expand.
    - Verify that clicking and dragging on a vertex causes the triangle's boundary to re-draw.
    - Verify that grading works.
    - Turn 'Show angle measures' and 'Show side measures'.
        - Verify that the angles and sides change as you modify the polygon.
- Create an [interactive-graph](http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22%5B%5B%E2%98%83%20interactive-graph%202%5D%5D%22%2C%22images%22%3A%7B%22https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2Fda8df81c78b22f5c69d477d8eabfb583968eaf84.png%22%3A%7B%22width%22%3A400%2C%22height%22%3A70%7D%2C%22https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2Fb59fc02ca1aae800977b8793ed22f647a1aa75ee.png%22%3A%7B%22width%22%3A425%2C%22height%22%3A150%7D%7D%2C%22widgets%22%3A%7B%22interactive-graph%202%22%3A%7B%22type%22%3A%22interactive-graph%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22step%22%3A%5B2%2C2%5D%2C%22backgroundImage%22%3A%7B%22url%22%3Anull%2C%22scale%22%3A1%2C%22bottom%22%3A0%2C%22left%22%3A0%7D%2C%22markings%22%3A%22graph%22%2C%22labels%22%3A%5B%22x%22%2C%22y%22%5D%2C%22showProtractor%22%3Afalse%2C%22showRuler%22%3Afalse%2C%22rulerLabel%22%3A%22%22%2C%22rulerTicks%22%3A10%2C%22range%22%3A%5B%5B-20%2C20%5D%2C%5B-20%2C20%5D%5D%2C%22graph%22%3A%7B%22type%22%3A%22polygon%22%2C%22numSides%22%3A%22unlimited%22%2C%22snapTo%22%3A%22grid%22%7D%2C%22correct%22%3A%7B%22type%22%3A%22polygon%22%2C%22numSides%22%3A%22unlimited%22%2C%22coords%22%3Anull%2C%22snapTo%22%3A%22grid%22%7D%7D%2C%22version%22%3A%7B%22major%22%3A0%2C%22minor%22%3A0%7D%7D%7D%7D%2C%22answerArea%22%3A%7B%22type%22%3A%22multiple%22%2C%22options%22%3A%7B%22content%22%3A%22%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%22calculator%22%3Afalse%7D%2C%22itemDataVersion%22%3A%7B%22major%22%3A0%2C%22minor%22%3A1%7D%2C%22hints%22%3A%5B%5D%7D) with type 'polygon' and click-to-add-points enabled.
    - Verify that clicking on the graph creates a bound.
    - Verify that making multiple points and clicking on an existing point closes the polygon.
    - Verify that dragging a point from an open polygon into the trashcan deletes it.
    - Verify that dragging a point from a closed polygon into the trashcan deletes it and re-opens the polygon.

Reviewers: aria

Reviewed By: aria

Differential Revision: http://phabricator.khanacademy.org/D13946