Add better options to get previous or next lesson period

• Add properties to LessonPeriod model
• Also annotate year while annotating a week
• Fix use of negative indexes in next_lesson

added part::backend type::bug type::feature labels

mentioned in merge request AlekSIS-App-Alsijil!56 (merged)

added 1 commit

• c2ae7474 - Apply 1 suggestion(s) to 1 file(s)

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resolved all threads

added 1 commit

• 7e4aa739 - Apply 1 suggestion(s) to 1 file(s)

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merged

mentioned in commit 0055d180