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.
Yea, we've been struggling with nomenclature throughout the process. I've proposed  - well to Xavier at least hopefully others were looped in:

- Score: the value emitted from xBlocks (.e.g. 1 point out of 3)
- Grade (aka Percent Grade): The application of scores to  the grading policy to produce a numeric grade, like 85%
- Letter Grade: Mapping of the Grade to a US-centric bucket, e.g. "85% = B"