TIM-125: Students completing scoring see results

* When a score becomes available for a student (i.e., when they have
  completed the minimum number of peer assessments and have received
  enough assessments to their submission), it is shown to the student.
* XXX: As an interim fix to not having thought through all the cases
  around multiple existing submissions, this commit disallows making
  multiple submissions.
* Refactoring of XBlock view to devolve functionality into smaller
  pieces - this is interim work until there's a better workflow layer
  (or perhaps it can be the prep for that.)
* TODO: since overgrading should be baked in, we should still show them
  their assessment page.