fix(auth): refactor current user handling into service

[czosel]

Writing instances of JS classes directly to the data property of the ember simple auth service is problematic, because ember simple auth always saves the serialized form of the class in local storage. In our case, this caused issues when the application is open in multiple tabs, and one tab triggers a token refresh: In that case, the initialization of the class instance is not triggered, and the serialized data is read directly from local storage.

This refactors the current implementation to use a separate service, as suggested by the ember-simple-auth docs: https://github.com/mainmatter/ember-simple-auth/blob/master/guides/managing-current-user.md