Idea: `notify` mandate earlier warning

[Bumber64]

notify warns the player when a production mandate is within 1 month of expiring. This is fine in most cases, but the player may desire a longer heads up.

I don't think notify presently supports configuring numbers, so it may be easier to just add a separate early warning option. Not sure what mandates typically start at, but a 3 month warning might be good.