Prevent expiration timer update on group update

It occurs when a message with a different expiration time is received.

The issue report highlights the scenario of a member leaving a group
(group update [quit] sent with expiration time = 0).

Fix https://github.com/WhisperSystems/Signal-Android/issues/5996
Fix https://github.com/WhisperSystems/Signal-iOS/issues/1566

js/models/messages.js

@@ -405,7 +405,7 @@
                             message.set({expireTimer: dataMessage.expireTimer});
                         }
 
-                        if (!message.isEndSession()) {
+                        if (!message.isEndSession() && !message.isGroupUpdate()) {
                             if (dataMessage.expireTimer) {
                                 if (dataMessage.expireTimer !== conversation.get('expireTimer')) {
                                   conversation.updateExpirationTimer(