Fixed calculation of mention count for direct channels (#3831)

* Fixed clientside calculation of direct channel mention count

* Fixed serverside calculation of direct channel mention count

store/sql_user_store.go

@@ -982,7 +982,7 @@ func (us SqlUserStore) GetUnreadCount(userId string) StoreChannel {
 	go func() {
 		result := StoreResult{}
 
-		if count, err := us.GetReplica().SelectInt("SELECT SUM(CASE WHEN c.Type = 'D' THEN (c.TotalMsgCount - cm.MsgCount) ELSE 0 END + cm.MentionCount) FROM Channels c INNER JOIN ChannelMembers cm ON cm.ChannelId = c.Id AND cm.UserId = :UserId", map[string]interface{}{"UserId": userId}); err != nil {
+		if count, err := us.GetReplica().SelectInt("SELECT SUM(CASE WHEN c.Type = 'D' THEN (c.TotalMsgCount - cm.MsgCount) ELSE cm.MentionCount END) FROM Channels c INNER JOIN ChannelMembers cm ON cm.ChannelId = c.Id AND cm.UserId = :UserId", map[string]interface{}{"UserId": userId}); err != nil {
 			result.Err = model.NewLocAppError("SqlUserStore.GetMentionCount", "store.sql_user.get_unread_count.app_error", nil, err.Error())
 		} else {
 			result.Data = count