Added util command to postprocess hektor export for exercises

An example:

./manage.py process_ilias_exercise ~/grady-data/info-1-uebung8.json /home/robin/Downloads/1578407820__9310__tst__results_129758.csv  ~/grady-data/grady-uebungen-teilnehmer.csv test.json --previous-hektor-files ~/grady-data/info-1-uebung5.json ~/grady-data/info-1-uebung6.json

when passing --sent-mail, an email is automatically sent to every new user