Migrate more strings to translation keys (#11637)

src/components/structures/NotificationPanel.tsx

@@ -58,8 +58,8 @@ export default class NotificationPanel extends React.PureComponent<IProps, IStat
     public render(): React.ReactNode {
         const emptyState = (
             <div className="mx_RightPanel_empty mx_NotificationPanel_empty">
-                <h2>{_t("You're all caught up")}</h2>
-                <p>{_t("You have no visible notifications.")}</p>
+                <h2>{_t("notif_panel|empty_heading")}</h2>
+                <p>{_t("notif_panel|empty_description")}</p>
             </div>
         );