Added is_active condition to Students and Teachers stats on the dashboard

Addresses a problem where the dashboard and arbitrary user lists counts didn't sync up, since the latter implicitly only searches for active users. (Presumably the discrepancy is if someone registers or makes a class and then requests an account deactivation; the class or registration doesn't go away. I have not addressed this, however, since that's not the issue I'm dealing with.) 

The change I made makes the assumption that in the Students and Teacher stats are all of the form "Number of students who..." or "Number of teachers who..."; in particular, the dict returned by prog.students() and prog.teachers() (as processed by JSONDataModule.stats()) should exclusively contain QuerySets from the ESPUser model, or else bad things may happen. If this is an issue, I can rewrite the code to check if this is true or not.
This `.filter` should be inside the `len()`.  (And I'll take that as a cue for a friendly reminder to test your code -- my guess is that that's an AttributeError and prevents all the dashboard stats from loading.)