Concurrent lock: Fix ownership handler in the backoffice

- Lost lock message will only be displayed once instead of being stacked every 60s 🤪
- Action buttons (except "Cancel") are disabled when the lock expires (regression from 3.0.0)
- Modal window is now larger

pages/ajax.render.php

@@ -2394,6 +2394,7 @@ EOF
 					}
 				}
 			}
+			$oPage->SetContentType('application/json');
 			$oPage->add(json_encode($aResult));
 			break;