Added ability to use HTML LightChild in question blocks

Html Light Child support for MCQ/MRQ choices
Thanks for the comment, it would be hard to understand otherwise. Where does the second div (the one below `.xblock-light-child`) comes from? 

Also, it's better to avoid using tag names in CSS and use just class names.

It's already a bit hacky, it would be good if the DOM could be updated to allow to only have to set it on:

```
mentoring .choices-list .choice-text > .xblock-light-child
```

Or, failing that, to:

```
mentoring .choices-list .choice-text > .xblock-light-child,
mentoring .choices-list .choice-text > .xblock-light-child > .xblock-html
```

(Or any other class name that would make sense to set for that second div, depending on where it comes from)