Add reusable <Modal> component, use with <Alert>

ts/components/ConfirmationDialog.tsx

@@ -1,4 +1,4 @@
-// Copyright 2019-2020 Signal Messenger, LLC
+// Copyright 2019-2021 Signal Messenger, LLC
 // SPDX-License-Identifier: AGPL-3.0-only
 
 import * as React from 'react';
@@ -28,6 +28,7 @@ function focusRef(el: HTMLElement | null) {
   }
 }
 
+// TODO: This should use <Modal>. See DESKTOP-1038.
 export const ConfirmationDialog = React.memo(
   ({ i18n, onClose, cancelText, children, title, actions }: Props) => {
     React.useEffect(() => {