- Only import substitutions for the current term
- Only import supervision substitutions related to a supervision
- Set subject to None if lesson is cancelled

- Use it for lessons and supervisions

The match of subjects is done case-insensitive and by the short name.
In order to set the subject the group form from the core is extended by this app.

Also add config option to use class groups instead of building course groups.

- Save references to term, lesson id and element number
- Update existing lessons
- Add logging

- Add configuration options

- Configurable force sync

- Move importers for common data to own file
- Move util code to util.py
- Add typing for methods

- Add refs for every type for matching UNTIS id to Chronos id