Will/use real classifiers when simulating errors

I tried using the `simulate_ai_grading_error` management command to test the "retry unfinished grading tasks" button.  Although grading tasks were rescheduled, they immediately failed because the command was using dummy data for the classifier.

In order to use this command -- especially in perf tests -- we need to actually train classifiers using EASE.  This will allow us to test how the system holds up when we reschedule many failed grading tasks.

@stephensanchez