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.

OK. It'd be good to confirm that this doesn't do more courseware tree traversals. Thx.