You'll never guess who noticed our highlighting bug!

Summary:
Er, maybe you will--I did? We weren't highlighting
widgets if the user left them empty anymore, since
we weren't re-rendering the widget containers when
a new highlight happened (since the content hadn't
changed, so we were caching the react output).

Test Plan:
 * open test.html
 * score the question without interacting with anything
 * see the widget be highlighted in orange
 * put a breakpoint in the renderer's render function
   where the "re-parse markdown and rerender everything"
   happens.
 * see this get triggered when I start interacting with
   the widget (as it unhighlights)
 * but then see this not get triggered again as I continue
   interacting with the widget

Auditors: michelle