Updating the studio mixin to deliver context with initial rendering

* Removed CodeMirror (just text fields now)
* Changed collapse / expand to use the is--hidden toggled class for assessments
* Broke oa_edit.html into several templates (this will help with drag and drop)
* Ripped everything out of oa_edit.js load()

Some things still broken:
* Certain buttons / listeners are not wired where they were in the load() function
* Save is not working due to criteria count in the Rubric

I'd prefer for 'criteria' and 'feedbackprompt' to be keys in the context itself, rather than within 'rubric'.  That more closely aligns with the XBlock definition, and it's consistent with the changes I've made in the refactoring PR I merged this morning.