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

That's a good question. I think I'll try adding {% spaceless %} to our editing templates to try and avoid any unwanted spacing.