Setup flow for cross-signing on login / registration

Still outstanding:
 * Keep password from login / registration
 * Confirmation on skip button

Fixes https://github.com/vector-im/riot-web/issues/11902

src/components/structures/auth/E2eSetup.js

@@ -0,0 +1,48 @@
+/*
+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 React from 'react';
+import PropTypes from 'prop-types';
+import AsyncWrapper from '../../../AsyncWrapper';
+import * as sdk from '../../../index';
+
+export default class E2eSetup extends React.Component {
+    static propTypes = {
+        onFinished: PropTypes.func.isRequired,
+    };
+
+    constructor() {
+        super();
+        // awkwardly indented because https://github.com/eslint/eslint/issues/11310
+        this._createStorageDialogPromise =
+            import("../../../async-components/views/dialogs/secretstorage/CreateSecretStorageDialog");
+    }
+
+    render() {
+        const AuthPage = sdk.getComponent("auth.AuthPage");
+        const AuthBody = sdk.getComponent("auth.AuthBody");
+        return (
+            <AuthPage>
+                <AuthBody header={false}>
+                    <AsyncWrapper prom={this._createStorageDialogPromise}
+                        hasCancel={false}
+                        onFinished={this.props.onFinished}
+                    />
+                </AuthBody>
+            </AuthPage>
+        );
+    }
+}