feat(client): Migrate OfflineWarning to TS (#42575)

* Rename OfflineWarning to tsx.

* Convert offline-warning to Typescript.

* Avoid using NodeJS.Timeout as type.

client/src/components/OfflineWarning/index.js

@@ -1 +0,0 @@
-export { default } from './OfflineWarning';

client/src/components/OfflineWarning/index.tsx

@@ -0,0 +1 @@
+export { default } from './offline-warning';

client/src/components/OfflineWarning/offline-warning.tsx

@@ -1,16 +1,20 @@
 import React from 'react';
-import PropTypes from 'prop-types';
 import { useTranslation } from 'react-i18next';
 
 import './offline-warning.css';
 
 const delayInMilliSeconds = 5000;
-let id;
-const propTypes = {
-  isOnline: PropTypes.bool.isRequired,
-  isSignedIn: PropTypes.bool.isRequired
-};
-function OfflineWarning({ isOnline, isSignedIn }) {
+let id: ReturnType<typeof setTimeout>;
+
+interface OfflineWarningProps {
+  isOnline: boolean;
+  isSignedIn: boolean;
+}
+
+function OfflineWarning({
+  isOnline,
+  isSignedIn
+}: OfflineWarningProps): JSX.Element | null {
   const { t } = useTranslation();
   const [showWarning, setShowWarning] = React.useState(false);
 
@@ -33,6 +37,5 @@ function OfflineWarning({ isOnline, isSignedIn }) {
 }
 
 OfflineWarning.displayName = 'OfflineWarning';
-OfflineWarning.propTypes = propTypes;
 
 export default OfflineWarning;