Keep registration spinner inside the auth modal

The spinner was taking on the full height of the modal and escaping off the
page. This keeps it contained inside the modal.

Fixes https://github.com/vector-im/riot-web/issues/8661

src/components/structures/auth/Registration.js

@@ -495,7 +495,9 @@ module.exports = React.createClass({
                 poll={true}
             />;
         } else if (this.state.busy || !this.state.flows) {
-            return <Spinner />;
+            return <div className="mx_AuthBody_spinner">
+                <Spinner />
+            </div>;
         } else {
             let onEditServerDetailsClick = null;
             // If custom URLs are allowed and we haven't selected the Free server type, wire