Get Classes Summary

Summary statistics for every class of this teacher: students, exercises completed, active students.

The numbers are the CLASS's, not its students': a devoir counts for the class it was given to, work done on the student's own initiative counts for the class they were in at that moment, and everything is bounded by the class's school year. So a class created today starts at zero even if its students have been working all year elsewhere, and a past class keeps the numbers of its year. See domain/class_activity.py.

period / school_year are still accepted and echoed back: they are what the client labels the numbers with.