Add @ prefix for sender username in push notifications (#11934)

* Add @ prefix for sender username in push notifications

* de-duplicating code

app/notification.go

@@ -640,7 +640,7 @@ type postNotification struct {
 func (n *postNotification) GetChannelName(userNameFormat string, excludeId string) string {
 	switch n.channel.Type {
 	case model.CHANNEL_DIRECT:
-		return n.sender.GetDisplayName(userNameFormat)
+		return n.sender.GetDisplayNameWithPrefix(userNameFormat, "@")
 	case model.CHANNEL_GROUP:
 		names := []string{}
 		for _, user := range n.profileMap {
@@ -670,7 +670,7 @@ func (n *postNotification) GetSenderName(userNameFormat string, overridesAllowed
 		}
 	}
 
-	return n.sender.GetDisplayName(userNameFormat)
+	return n.sender.GetDisplayNameWithPrefix(userNameFormat, "@")
 }
 
 // addMentionedUsers will add the mentioned user id in the struct's list for mentioned users