These methods should be moved out of the User class.
- changeableByGroup - a simple utility wrapper around config variables, no dependencies
- changeableGroups - depends on UserGroupManager and on PermissionManager. This is problematic, since PermissionManager depends on UserGroupManager, thus moving the method into UGM would create a cyclic dependency. Alternatives: wait for authority object? create a higher-level service depending on both PermManager and UGM. Lazy-load PermissionManager for now.