mattdrayer/api-proforma-to-gradebook: Calculate/store proforma grade in ...

@martynjames @chrisndodge @ziafazal -- I went ahead and added proforma grade calculation/storage to the gradebook as outlined in the Jira ticket assigned to Zia.  I've done this in order to set the gradebook up for individual PR back to master, which needs to happen ahead of the PR for api_manager due to some dependencies that have been introduced in some of the views.

the grading work is already committed and in use -- moving proforma here won't add too much in the way of overhead, I think, because it's just doing some additional computation on the info returned by grades.grade.  So yes, agreed that we should push the whole thing to Celery, but this particular change shouldn't make a noticeable impact.