msglist [nfc]: Renumber a TODO about muting

We already handle muted topics, but we still need to update the
message list on muting/unmuting a channel, which is zulip#1255.

Author: chrisbobbe

lib/model/channel.dart

@@ -322,7 +322,7 @@ class ChannelStoreImpl with ChannelStore {
           case SubscriptionProperty.color:
             subscription.color                  = event.value as int;
           case SubscriptionProperty.isMuted:
-            // TODO(#421) update [MessageListView] if affected
+            // TODO(#1255) update [MessageListView] if affected
             subscription.isMuted                = event.value as bool;
           case SubscriptionProperty.inHomeView:
             subscription.isMuted                = !(event.value as bool);

lib/model/message_list.dart

@@ -1132,7 +1132,7 @@ class MessageListView with ChangeNotifier, _MessageSequence {
       case MentionsNarrow():
       case StarredMessagesNarrow():
         // The messages didn't enter or leave this narrow.
-        // TODO(#421): … except they may have become muted or not.
+        // TODO(#1255): … except they may have become muted or not.
         //   We'll handle that at the same time as we handle muting itself changing.
         // Recipient headers, and downstream of those, may change, though.
         _messagesMovedInternally(messageIds);