/ta set king <nation> <player/npc> Functionality

This addresses half of issue 1517, adding the ability for an admin to set a a player or NPC as king by setting them as mayor of the capital city.

One thing to address is the upkeep of a nation when replacing the king with an NPC; with towns, this disables a nation's upkeep, but as the methods are not available for nations at the moment, only the capital town's upkeep is disabled when an NPC is made king.

I had to revert some commits and reapply them because of some accidental file changes, so six out of the seven commits didn't have an impact.