Added ability to use HTML LightChild in question blocks

Html Light Child support for MCQ/MRQ choices
It was some time ago that I made this, but I think the purpose of the line was to make sure the 'Try Again' button was clickable. This was probably not the best way to do that (after all, the error message that would result would not be very informative). The error you received may have been due to it not being clickable or something. Because if it wasn't clickable, and an error bubbled up, Django's transaction management may have been invoked for some reason to try to undo any current work. This might have happened if Xblock-SDK was running under Django 1.7, since I believe that Django 1.7 has views in their own transactions by default. But I'm not sure.