Fix and reorder imports

aleksis/apps/untis/forms.py

aleksis/apps/untis/management/commands/untis_import_xml.py

aleksis/apps/untis/model_extensions.py

aleksis/apps/untis/settings.py

aleksis/apps/untis/util/mysql/importers/absences.py

aleksis/apps/untis/util/mysql/importers/common_data.py

aleksis/apps/untis/util/mysql/importers/events.py

aleksis/apps/untis/util/mysql/importers/holidays.py

aleksis/apps/untis/util/mysql/importers/lessons.py

aleksis/apps/untis/util/mysql/importers/substitutions.py

aleksis/apps/untis/util/mysql/main.py

aleksis/apps/untis/util/mysql/util.py

aleksis/apps/untis/views.py