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.
Moving this inside the loop seems incorrect? Not wrong, but better to have it be calculated before the loop begins.

Also, I think I'd be tempted to collect the value as a bool to begin with, so the conversion only has to happen once:
e.g. ```include_course_counts = str2bool(request.QUERY_PARAMS.get('include_course_counts', False))```

then the expression becomes ```if include_course_counts:``` which feels more readable