WIP commit

* Back-end refactoring of OE 'save_answer' so that when
  SAVEONLYSKIPGRADING is set True in the data posted, it saves the
  response without submitting to the grader.

* I'm not sure this is actually a reasonable approach; I don't feel
  confident it will work as intended. But it gives us something to try
  that we can experiment with a little.

* Needs UI to do anything.