Fix "Add image" button

Summary:
react was optimizing away a full re-render of the StatefulEditorPage
component, which meant it never copied the image url provided by the
add-image button to its state.

The server call was succeeding, but the returned URL ended up in
the StatefulEditorPage's props, but not state. Now it should end
up in both places.

It's likely that add-image should actually be calling setState or
some other update function on the StatefulEditorPage, but this
is a smaller fix to address the immediate bug.

Test Plan:
 * make all to put this into webapp
 * edit webapp to put a fake URL in if the ajax request fails
   (I must not have the right secrets set up)
 * Add an image in webapp, and see if a URL is put in the
   question editor box

Auditors: joel