Decouple annotation storage from user management

[reckart]

Describe the refactoring action Decouple the annotation storage from user management.

Expected benefit There are reasons to store annotations for entities that are no actually users, e.g. for agents, curators, or even the initial CAS. It is mixing up concerns that we currently treat all of these as (virtual) users. It would be better if there were a proper separation of concerns between annotation management and user management.