Reimplement notification system on base of new calendar system

Possible Triggers:

• Lesson event created (e. g. extra lesson, event, whatever → regular lessons have to be excluded)
• Lesson event changed (⬆)
• Lesson event deleted (⬆)
• Lesson event created which amends something (e. g. substitution, supervision substitution)
• Lesson event changed which amends something (⬆)
• Lesson event deleted which amended something (⬆)

This ultimately leads to track all LessonEvent stuff and exclude those events which represent regular lessons.

Alarms shall be created if

• a new extra lesson, event, substitution, supervision substitution whatever is created (no earlier than a certain point x before this event → create alarm)
• one of those events changed (only if initial notification has been sent, otherwise initial alarm has to be updated)
• one of those events was deleted (only if initial notification has been sent, otherwise initial alarm has to be deleted)

The notification system for calendar events should depend on VALARM entries.

The calendar event should have some kind of API for custom notifications where Chronos can add some extra logic for the alarms.