Renderer: Fix crash on adding groups in articles

Summary:
Missing widgetInfo when a group is first created was causing
the group widget's interWidgets call to crash. (It was calling
interWidgets asking for all widgets of type "group", but the
widgetInfo was undefined, so checking widgetInfo.type on line
345 was crashing).

@joel you win audit roulette!

Test Plan:
 * Add a group to /article.html (and see it doesn't crash)
 * Add text to the group (and see it doesn't crash)
 * Try adding an input-number to test.html and scoring it,
   and see that orange highlighting still works.

Auditors: joel