allow a setting controller to validate and revert a change (asynchronously)

src/settings/controllers/SettingController.js

@@ -39,6 +39,10 @@ export default class SettingController {
         return null; // no override
     }
 
+    canChangeTo(level, roomId, newValue) {
+        return Promise.resolve(true);
+    }
+
     /**
      * Called when the setting value has been changed.
      * @param {string} level The level at which the setting has been modified.