sorting happens in python now - since only this solution is compatible
with prefetch

Is used in person_overview & coursebook.

The previous filtering result for participations did not survive the
annotations done by annotate_person_statistics_for_school_term.

& reintroduce some abbreviations.

It's now reachable under coursebook/print/group/id

django.po still needs adjusting

This reverts commit b3c63074.