Add separate component for post-auth security flows

Instead of twisting `AuthBody`, this adds a new component for the different
styling of post-auth security flows. This also makes them fixed width and
adjusts padding to match designs.

res/css/views/auth/_AuthBody.scss

@@ -1,5 +1,6 @@
 /*
 Copyright 2019 New Vector Ltd
+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.
@@ -15,6 +16,9 @@ limitations under the License.
 */
 
 .mx_AuthBody {
+    width: 500px;
+    font-size: 12px;
+    color: $authpage-secondary-color;
     background-color: $authpage-body-bg-color;
     border-radius: 0 4px 4px 0;
     padding: 25px 60px;
@@ -92,16 +96,6 @@ limitations under the License.
     }
 }
 
-.mx_AuthBody_noHeader {
-    border-radius: 4px;
-}
-
-.mx_AuthBody_loginRegister {
-    width: 500px;
-    font-size: 12px;
-    color: $authpage-secondary-color;
-}
-
 .mx_AuthBody_editServerDetails {
     padding-left: 1em;
     font-size: 12px;