Add helper methods for getting next/previous lesson of a person on a day

added part::backend type::feature labels

mentioned in merge request AlekSIS-App-Alsijil!98 (merged)

added 1 commit

• 3fb43b95 - Let next_lesson/prev_lesson base on common method _adjacent_lesson

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resolved all threads

@nik Please review again.

merged

mentioned in commit 2ca85d8a