ref #4066 - add 404-error handling
ref #4066 - add info message (refactor)
ref #4066 - add mail templates for unsubscribe
ref #4021 :construction: get system messages from the corresponding system
ref #4066 - minor changes
general - fix the  god damn BeanNotOfExpectedTypeException
ref #4021 - systems wont load per system due to a TransactionSystemException (query returns a incident instead of a maintenancewindow and reverse)
ref #4066 - add mail function for future use if needed
ref #4066 - update subscriber implemented but not working