Add modal look to authentication flows

This changes the auth screens to use the modal-like style of the redesign.

This does not attempt to style the actual body content of each screen.  Instead,
it covers the header area with logo, footer links, and overall modal container
only.

src/components/views/auth/AuthBody.js

@@ -0,0 +1,27 @@
+/*
+Copyright 2019 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.
+*/
+
+'use strict';
+
+import React from 'react';
+
+export default class AuthBody extends React.PureComponent {
+    render() {
+        return <div className="mx_AuthBody">
+            { this.props.children }
+        </div>;
+    }
+}