New notifications for timeout warnings

It’s annoying to lose a game by timeout because you forgot to move. I’ve added a new notification type (on by default) for games with a per-move limit of 24 hours or more, warns you when only about 10% of your time remains, rounded up to a number of hours.

Examples:

• for a 24 hour-per-move game, you’ll get a warning when you only have 3 hours left to make a move (10% of 24 = 2.4 hours, rounded up to 3)
• for a 36 hour-per-move game, you’ll get a warning when you only have 4 hours left to make a move (10% of 36 = 3.6 hours, rounded up to 4)