Automatic feedback selection, config cleanup
Features:
* choose-feedback
* Implemented automatic and random feedback selection based on feedbacks received. Students who have received less feedbacks on submissions and reviews are more likely to be selected.
* save-feedback
* Added save mechanism to CSV for the count of feedbacks a student has received.
* Integrated save-feedback command as prompt into mail command.
Config and documentation changes:
* Moved the prompt of some properties into the ConfigHandler and added backing fields to avoid code duplication.
* Name of Moodle authorization cookie can now be set through the ConfigHandler.
* Renewed readme.md with new functionality, better readability and tutorial for shared tutor work.
