Use scroll bars on horizontal overflow of clue boxes

Long category names may cause horizontal overflow. To prevent the boxes from bleeding out, add a scroll bar and adjust the size of the the clue board.

- Adjust the width of the clue board to match the total width of the category boxes on load
- Set the overflow-x style property to "auto" for the main board
- Buttons are not disabled when the clue box is showing (this small feature can be implemented should the need arise in the future)