Highlight active unit in studio container page's outline

TNL-291

@andy-armstrong @cahrens please review.

@talbs thanks for the front-end work :smile: 

@explorerleslie would you like me to create a sandbox?
Hmm, maybe I'm just seeing it in `course_outline_modal.js`... `git grep`ping for getContext returns a lot, but I'm now realizing many of those hits are in common or vendor files. I'm happy to change to `getTemplateContext` in this case.