Two bug fixes

Fixes:

> switching between two items that both have an input-number widget in
> the answer area where the correct answer from one item is retained in
> the UI when switching to the next item.

> edit an exercise with multiple items, then switch between items by
> clicking them on the left while just staying in the edit tab on the
> right. When you switch between items, the preview of the question and
> answer area doesn't update (though the hints seem to)

The problem in the first case is that the input used `defaultValue`
rather than `value`.

In the second case the EditorPage didn't update its renderer when it was
updated.

Test Plan:
Edit /math/geometry/segments-and-angles/intro_euclid/e/drawing-lines.
Switch between items and note that the renderer updates ever time. Also
switch two items to use a number input - one with value 1 and the other
with value two. Note that switching between them toggles the "Correct
answer" field between 1 and 2.

Auditors: alex, eater