Together with 18544e06, updating the existing
migration is safe, because

 a) if the migration failed somewhere due to the multiple leaf nodes, it was
    never applied, and changing it is safe;
 b) if it succeeded, it will be renamed to 0008_ later, and thus
    re-applied

Needed in Alsijil etc.

Add additional rules&permissions in order to differentiate different access to individual timetables

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