Have yard markers "stick" to the top edge

When you are zoomed in and working, it's really hard to see what yard number you're on.  We should have the yard marker be "sticky" and track down the field, sort of like the way that in a spreadsheet you can lock the table headers.

I don't think we need to do the bottom or the horizontal lines.  We have hashes for the horizontal, and I think people will look to the top naturally.