Prevent lesson edition in the future

Please disallow entering data for lessons in the future, i.e. lessons that have not yet started at the current point in time.

It is important to do this in a way that the LessonDocumentation object and the PersonalNote objects are not even created if a lesson in hte future is opened, to avoid issues when a new timetable is imported that overwrites lessons that have already documentatiosn attached.