Extract auth header logo to new component

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

res/css/_components.scss

@@ -27,6 +27,7 @@
 @import "./views/auth/_AuthButtons.scss";
 @import "./views/auth/_AuthFooter.scss";
 @import "./views/auth/_AuthHeader.scss";
+@import "./views/auth/_AuthHeaderLogo.scss";
 @import "./views/auth/_AuthPage.scss";
 @import "./views/auth/_InteractiveAuthEntryComponents.scss";
 @import "./views/auth/_ServerConfig.scss";

res/css/views/auth/_AuthHeader.scss

@@ -19,11 +19,3 @@ limitations under the License.
     padding: 25px 50px;
     box-sizing: border-box;
 }
-
-.mx_AuthHeader_logo {
-    margin-top: 15px;
-}
-
-.mx_AuthHeader_logo img {
-    width: 100%;
-}

res/css/views/auth/_AuthHeaderLogo.scss

@@ -0,0 +1,23 @@
+/*
+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.
+*/
+
+.mx_AuthHeaderLogo {
+    margin-top: 15px;
+}
+
+.mx_AuthHeaderLogo img {
+    width: 100%;
+}