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

res/css/views/auth/_AuthBody.scss

@@ -103,3 +103,7 @@ limitations under the License.
     text-align: center;
     width: 100%;
 }
+
+.mx_AuthBody_spinner {
+    margin: 1em 0;
+}