Resolve "Break supervisions must be deleted cleanly"

By using get_or_create with teacher in defaults for supervisions, supervisions weren't deleted if there was any (other/new) supervision in the same area, same break and same validity range. The reason for this is that there is a piece of code at the end of the importer which deletes all supervisions which weren't created in the lines with get_or_create. By using teacher as a normal get argument, new supervisions are correctly created and old ones are correctly detected as old ones which have to be deleted.

Closes #21 (closed)