get_or_create for lesson documentations and personal notes doesn't work correctly because of missing unique index

Opening a single lesson (or event or extra lesson) often causes an Internal Server Error. Internally, using get_or_create sometimes raise a MultipleObjectsReturned exception. The reasons for this behavior are described here:

1. https://stackoverflow.com/a/29521117
2. https://adriennedomingus.com/blog/the-perils-of-getorcreate-race-conditions

A working solution is adding an unique index for the fields lesson_period, week, year, event and extra_lesson. This combination will always be unique.