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'm a little concerned about using `ASSESSMENT_STEP_NAMES` for two different purposes here.  Right now, the assessment step names happen to not include dashes, but that's not guaranteed by the workflow API.  If someone were to add a new assessment type that included dashes, this wouldn't work.

It might be cleaner here to just replace dashes with underscores.