Add mw-echo-payload class to body in Special:Notifications as well

This was already done earlier in the old flyout formatter.

Change-Id: I3b4029de4c76910e139d02c9b93cf50e0fac0236

includes/formatters/SpecialNotificationsFormatter.php

@@ -23,7 +23,7 @@ class SpecialNotificationsFormatter extends EchoEventFormatter {
 		if ( $body ) {
 			$html .= Xml::tags(
 					'div',
-					null,
+					array( 'class' => 'mw-echo-payload' ),
 					$body->parse()
 				) . "\n";
 		}