Two scenarios that this PR fixes:

• Player A offers a draw, B declines, 20 ply later A is eligible to offer again. However if at that point A goes back a few moves in the notation, the draw button becomes greyed out.
• Player A goes back a few moves in the notation (say 10 plies back) and offers a draw, which B declines. The players play around 10 plies more, at which point the draw button will be ungrayed for A. Clicking the button has no effect (the backend prevents the draw offer from reaching B), but this still isn't ideal.