refactor: date calculation when linked with contracts

fix: check if a user exists first before trying to delete him from QUIQQER groups when archiving namerobot/tool#35

feat: added Events
feat: introduced cancelEndDate field to have a specific end date

feat: Datumsformatierung nach Benutzer-Locale; Unterscheidung zwischen tag- und sekundengenauer Laufzeitberechnung