Student Notes: cleanups.

@jmclaus , @tymofij please review.
yes, but only when neither `is_pure_xblock` nor `is_lti_module` are truthy. 
`LTI` - I am not sure our scripts will work fine with LTI, but we can imagine LTI block be decorated, and we do not raise exception in that case, so seems things will just go wrong.
`is_pure_xblock` - might just happen, no?