mattdrayer/api-orgs-complete-count: New metrics operation

@dino-cikatic @dsego @martynjames -- new API operation to return the count of users who are considered 'grade complete', which is currently defined as all users in an organization having proforma grade <= actual grade.  I've included a small points buffer to account for potential scoring/averaging issues.

Take a look and let me know what you think.  If it does indeed get the job done from a functionality standpoint we can then explore bundling the data into another operation if it would be more useful.
Seems like we'll really want to filter this for a certain course_id. Indeed, for the whole organization it would be useful to get the response per course, rather than an organization-wide count