Cooldown for joining towns to prevent town-hopping

[Ekmot276]

Add an option (either config or permission-based like husktown husktowns.cooldown.join.600 which would mean a player can't join another town for 600 seconds after he joined another town.

For convenience this timer should start after the player joined a town and not after the left his old one because this would lead to high waiting-times when a player wants to switch his town at some point.

It might also be reasonable to trigger the cooldown when a player creates his own town as well.