usermanager: Add back temporary is_admin to warn about deprecated API usage Goal: Introduce role-auth with minimal disruption is_admin() is unsafe in a system with per-session permissions, so it has been deprecated. Roll-out approach: 1) First, log a warning when is_admin() is used. It should continue to function normally, backed by the new role API. Nothing is really using per-session authz yet, so there is minimal security concern. The 'strict_deprecate_is_admin' global setting can be set to 'true' to force a hard failure of is_admin() attempts (it will log an error and always return false). 2) In some time (at least 1 week), but possibly longer depending on the number of affected deployments: switch 'strict_deprecate_is_admin' to 'true' by default. It can still be disabled for systems that need it. 3) Further in the future, before the next release, the option will be removed and is_admin() will be permanently disabled.