Next/prev functions for persons don't return the next/prev lesson

The next/prev functions for persons don't return the next/previous lesson, but some other lesson of the person on the same day. That suggests that the query is wrongly ordered.

added part::backend prio::1 type::bug labels

created merge request !143 (merged) to address this issue

mentioned in merge request !143 (merged)

mentioned in commit 7e1aaf78

closed with merge request !143 (merged)