[FIX] mail: do not update count if message in no channel

When a message is not part of any channel, the message counter is
incremented while the message won't appear in any channel. This leads to
an inconsistency between the number of messages displayed in the status
bar and the actual messages in Discuss.

opw-706884

addons/mail/static/src/js/chat_manager.js

@@ -479,7 +479,9 @@ function on_needaction_notification (message) {
         increment_unread: true,
     });
     invalidate_caches(message.channel_ids);
-    needaction_counter++;
+    if (message.channel_ids.length !== 0) {
+        needaction_counter++;
+    }
     _.each(message.channel_ids, function (channel_id) {
         var channel = chat_manager.get_channel(channel_id);
         if (channel) {