Improve Numeric Input

Summary: Changing over to using just props causes a few changes in behavior (and glitches). For example, if you have pi in one ungraded answer, and 1/2 in another wrong answer, and then change the wrong answer to be correct, it shifts positions (which I guess is fine), but for whatever reason it reverts to decimals. Yet, when you come back to the page (or click JSON), it'll change back to the proper format. Still investigating

Test Plan: still works, coded more nicely

Reviewers: jack

Reviewed By: jack

Differential Revision: http://phabricator.khanacademy.org/D8412