[MM-56006] Convert ./components/discard_changes_modal.tsx from Class Component to Function Component (#25620)

* MM-56006: Converted to React FC

* Update webapp/channels/src/components/discard_changes_modal.tsx

Co-authored-by: Daniel Espino García <larkox@gmail.com>

* lint changes

---------

Co-authored-by: Daniel Espino García <larkox@gmail.com>

webapp/channels/src/components/discard_changes_modal.tsx

@@ -12,16 +12,19 @@ type Props = {
     onCancel: (checked: boolean) => void;
 }
 
-export default class DiscardChangesModal extends React.PureComponent<Props> {
+const DiscardChangesModal = ({
-    public render(): JSX.Element {
+    show,
-        const title: JSX.Element = (
+    onConfirm,
+    onCancel,
+}: Props) => {
+    const title = (
         <FormattedMessage
             id='discard_changes_modal.title'
             defaultMessage='Discard Changes?'
         />
     );
 
-        const message: JSX.Element = (
+    const message = (
         <FormattedMessage
             id='discard_changes_modal.message'
             defaultMessage='You have unsaved changes, are you sure you want to discard them?'
@@ -29,7 +32,7 @@ export default class DiscardChangesModal extends React.PureComponent<Props> {
     );
 
     const buttonClass = 'btn btn-primary';
-        const button: JSX.Element = (
+    const button = (
         <FormattedMessage
             id='discard_changes_modal.leave'
             defaultMessage='Yes, Discard'
@@ -38,8 +41,6 @@ export default class DiscardChangesModal extends React.PureComponent<Props> {
 
     const modalClass = 'discard-changes-modal';
 
-        const {show, onConfirm, onCancel} = this.props;
-
     return (
         <ConfirmModal
             show={show}
@@ -52,5 +53,6 @@ export default class DiscardChangesModal extends React.PureComponent<Props> {
             onCancel={onCancel}
         />
     );
-    }
+};
-}
+
+export default React.memo(DiscardChangesModal);