mattdrayer/api-proficiency-redux: New StudentGradebook

@chrisndodge @martynjames @ormsbee @ziafazal -- complete rework of "proficiency" with the introduction of a new 'gradebook' app.  

Essentially, 'proficiency' as a concept is no more -- we simply have 'grades' -- you'll see the leaders list is maintained although there are some slight schema changes as well as URI changes which will require corresponding updates in Apros.  These changes are most-easily reviewed in this PR by looking at the response_data elements returned by the modified views.

The seed script is not included here at the moment -- I'll be adding it either to this PR or in a follow-on PR in the next couple of days.  I'm really just looking for feedback and hopefully a thumbs-up that the logic/math are sound and that all of Apros' needs are met with the updated (smaller) API.
Could this queryset be the size of the number of people in the entire course?