MDL-48510 inbound: Support disabling of expiration changes in UI

lib/classes/message/inbound/handler.php

@@ -140,6 +140,18 @@ abstract class handler {
         return $this->validateaddress = $validateaddress;
     }
 
+    /**
+     * Whether the current handler allows changes to expiry of the generated email address.
+     *
+     * By default this will return true, but for some handlers it may be
+     * necessary to disallow such changes.
+     *
+     * @return boolean
+     */
+    public function can_change_defaultexpiration() {
+        return true;
+    }
+
     /**
      * Whether this handler can be disabled (or enabled).
      *