Implement group email business rules on REST API

Due to code refactoring the 8.4 release always sends emails when adding/removing users from groups. The default settings from the olat.properties are discarded (role based definition if mail is mandatory or not)

The goal of this issue is to implement the default settings when using REST API. Mail notifications will be sent when the mail notification is mandatory and not send when it is not mandatory. The mail sending however can not be controlled using the API, the defaults will always be applied according to the users roles.