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

There was some exception handling here that would originally return with an error; if we did that, it would not allow the author to recover from having un-parseable examples. This would at least allow the examples to be rewritten. 

That being said, it may not be crucial; the XML parsing should be dropped in the next few stories.