Make the widgets minimizable

Summary:
Adds a chevron to the title container of a widget, then makes it "display: none/visible" the widget onClick of the title container. I spent a good hour trying to make it accordion, but evidently that's hard. I think I'll just wait until React implements that as an addon. I'll probably delete all (my hard work regarding) the React CSSTransition.

Demo GIF (click on it)
{F13644}

Test Plan:
1) Make sure that when it hides, you still have access to the JSON (which makes it render)
2) See that it's somewhat useful
3) Check transformer to make sure the isGraded prop box isn't ugly.

Reviewers: alpert

Reviewed By: alpert

CC: alpert, jack

Differential Revision: http://phabricator.khanacademy.org/D7039