Inline Analytics

  Display of answer distribution graphics inline on the problem page
  for instructors.

  Problem types supported for graphics:
    - MultipleChoiceResponse
    - ChoiceResponse

  Problem types supported for line item display:
    - OptionResponse
    - NumericalResponse
    - StringResponse
    - FormulaResponse
    via setting

Internal review only for now.
Since our first `if` in this branch is `if message != None`, doesn't
that mean that all subsequent branches can only be evaluated if
`message == None`? Assuming that, can we just change this to:
```python
% else:
```