Review and revise definition of Billable Users

[mkdir-washington-edu]

With what we're offering changing (e.g. 1: we get course rosters and instructors can assignment 0s to students who never launch) we should re-write and re-calculate the billable users definition in light of how we work now.

We can also revisit use cases we currently don't use to accure billable users (e.g. 2: an instructor creates annotations in June for a course taught in September, students view the annos in September but don't create their own) and decide if we want to account for those use cases or not.