Add admin_only setting

Closes #2979426

config/schema/quick_node_clone.schema.yml

@@ -20,6 +20,9 @@ quick_node_clone.settings:
     clone_status:
       type: string
       label: 'Cloned node status'
+    admin_only:
+      type: boolean
+      label: 'Make cloning available only to content administrators'
     create_group_relationships:
       type: boolean
       label: 'Create group relationships?'