Issue with teachings that fork

[Wornairz]

If at a certain academic year a teaching forks (e.g. the teaching was split into two channel A-L / M-Z), the schede before that academic year aren't provided by the insegnamento/with-id/<id>/schedeopis API.

[LemuelPuglisi]

insegnamento.schedeopis.withid actually fetch the opis data strictly connected to the course. We should move the issue on the right API affected by the same problem, insegnamento.coarse.schedeopis.