Added totals for different student tracks

sorted the output, easier to read

Add is_active filter to only generate for students enrolled

Adding back total_enrollment

Removing active filter

Certs will generate for all users in the course, not just those "active"

Typo

Renaming headings to make more sense

Renaming variables to match headings
Merge pull request #3621 from edx/han/cert-report

Added totals for different student tracks