Fix for responsive controls-row misalignment

The default style here (https://github.com/twbs/bootstrap/blob/c9789f75153539a0ca0a2f31234716f527024333/docs/assets/css/bootstrap.css#L424) is taking precedence over the responsive style because it has the row-fluid class, so I'm adding the row-fluid class to the responsive style to compensate. I discovered the issue when working on [this bounty source bug](https://www.bountysource.com/issues/639345-bountybox-buttons-misaligned).