Upgrade sanitize-html, set nesting limit

This uses the recently added option to allow specifying a nesting limit.

Fixes https://github.com/vector-im/element-web/issues/15122

src/@types/sanitize-html.ts

@@ -0,0 +1,23 @@
+/*
+Copyright 2020 New Vector Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import sanitizeHtml from 'sanitize-html';
+
+export interface IExtendedSanitizeOptions extends sanitizeHtml.IOptions {
+    // This option only exists in 2.x RCs so far, so not yet present in the
+    // separate type definition module.
+    nestingLimit?: number;
+}