Allow connecting to an IS from address picker

This allows those who previously disconnected from an IS to either choose the
default IS or a custom one from Settings via the address picker dialog.

Part of https://github.com/vector-im/riot-web/issues/10619

src/utils/IdentityServerUtils.js

@@ -0,0 +1,30 @@
+/*
+Copyright 2019 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 SdkConfig from '../SdkConfig';
+import MatrixClientPeg from '../MatrixClientPeg';
+
+export function getDefaultIdentityServerUrl() {
+    return SdkConfig.get()['validated_server_config']['isUrl'];
+}
+
+export function useDefaultIdentityServer() {
+    const url = getDefaultIdentityServerUrl();
+    // Account data change will update localstorage, client, etc through dispatcher
+    MatrixClientPeg.get().setAccountData("m.identity_server", {
+        base_url: url,
+    });
+}