Extract auth header logo to new component

This will allow `riot-web` to replace only the logo, rather than the whole
header.

src/components/views/auth/AuthHeader.js

@@ -18,16 +18,17 @@ limitations under the License.
 'use strict';
 
 const React = require('react');
+import sdk from '../../../index';
 
 module.exports = React.createClass({
     displayName: 'AuthHeader',
 
     render: function() {
+        const AuthHeaderLogo = sdk.getComponent('auth.AuthHeaderLogo');
+
         return (
             <div className="mx_AuthHeader">
-                <div className="mx_AuthHeader_logo">
-                    Matrix
-                </div>
+                <AuthHeaderLogo />
             </div>
         );
     },

src/components/views/auth/AuthHeaderLogo.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 AuthHeaderLogo extends React.PureComponent {
+    render() {
+        return <div className="mx_AuthHeaderLogo">
+            Matrix
+        </div>;
+    }
+}