Use user key instead of user name to identify users

[culmat]

The current implementation uses https://docs.atlassian.com/atlassian-user/2.0/apidocs/com/atlassian/user/Entity.html#getName()
We should use https://docs.atlassian.com/ConfluenceServer/javadoc/6.15.6/com/atlassian/confluence/user/ConfluenceUser.html#getKey-- instead for the following reasons

• only the id is stable, i.e. when changing login infrastructure user name can change
• com.atlassian.sal.api.user.UserKey is a class with type safety as opposed to user name of type String
• it will allow proper anonymisation see How to anonymize user's content stored by digital-signature macro #71 and Issue after anonymization #95

⚠️ consider backwards compatibility