UIU-2563 immutable permission is necessary to manipulate sets (#2102)

Manipulating run-time (mutable) sets composed of build-time (immutable)
permissions requires (drumroll) permission to manage immutable
permissions.

Refs UIU-2563

package.json

@@ -187,6 +187,7 @@
           "perms.permissions.item.post",
           "perms.permissions.item.delete",
           "settings.users.enabled",
+          "perms.users.assign.immutable",
           "perms.users.assign.mutable"
         ],
         "visible": true