letterrip: Allow the grid to grow beyond the default page width

letterrip: Allow the grid to grow beyond the default page width

It's silly to make the user scroll back and forth when the grid gets
wider than the default page. So set the board's container to use a
maximum width that is based on the viewport, rather than just being a
percentage of the parent. For this to work, we also have to drop the
parent-relative 100% maximum width from .board-and-rack.