usermanager: Remove concept of global authz provider Rationale: - Removes a bunch of code! - We don't have many cases where an actor is not bound to one of our hosts - A notable exception is the admin shell, but if we ever attempt to lock those sessions down, there is a load of other work that also has to be done. And it's not clear if we would need a global authz provider for that anyway. - Removes an extra edge case from the necessary mental model for operators - Sessions that aren't bound to a host generally are anonymous or have an alternative auth model (such as by IP addres). - With the encapsulation now provided by util.roles, ad-hoc "detached roles" can still be created anyway by code that needs them.