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
ref #4066 - minor change
ref #4066 - minor change
ref #4066 - minor change
Merge remote-tracking branch 'origin/gid-4066-unsubscribe' into prd/gid-4021/4066/4067-merged

Conflicts:
	sysinfos-api/src/main/java/ch/alv/sysinfos/api/query/FindByEmailQueryProvider.java
	sysinfos-api/src/main/java/ch/alv/sysinfos/api/web/config/ApplicationConfig.java
	sysinfos-api/src/main/java/ch/alv/sysinfos/api/web/config/ServicesConfig.java
	sysinfos-api/src/main/java/ch/alv/sysinfos/api/web/controller/BaseResourceController.java
	sysinfos-api/src/main/java/ch/alv/sysinfos/api/web/controller/SubscriptionController.java
	sysinfos-api/sysinfos-api.iml
	sysinfos-ui/public/i18n/messages_de.properties
	sysinfos-ui/public/i18n/messages_fr.properties
	sysinfos-ui/public/i18n/messages_it.properties
	sysinfos-ui/public/js/modules/sysinfos-common.subscriber.js
general - minor changes,  refactoring, and merging - add protractor test for #4066