Check for cross-signing homeserver support

This checks the homeserver to ensure it supports cross-signing (via the versions
feature flag) before attempting bootstrapping or offering user verification.

Fixes https://github.com/vector-im/riot-web/issues/11863

src/hooks/useAsyncMemo.js

@@ -0,0 +1,25 @@
+/*
+Copyright 2020 The Matrix.org Foundation C.I.C.
+
+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 { useState, useEffect } from 'react';
+
+export const useAsyncMemo = (fn, deps, initialValue) => {
+    const [value, setValue] = useState(initialValue);
+    useEffect(() => {
+        fn().then(setValue);
+    }, deps); // eslint-disable-line react-hooks/exhaustive-deps
+    return value;
+};