Necessary for matching method by subjects and parent groups (MySQL importer)

Resolve "Support import from MySQL"

Closes #4

See merge request !13

Also, throws exceptions on unrecoverable errors to terminate and rollback the transaction cleanly

- 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.