Introduce reputation command cooldown for repping players who are not online

[Edward Hart-Davis]

Otherwise, you'll get players who want to play with the reputation command, and see that they can type in random phrases and suddenly (good|bad)rep a random player who probably hasn't been around in a few years, over and over. Perhaps a cooldown of a few minutes?

[Anna Bakker]

add cooldown for using /rep on players that aren't ingame, resolves #416

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