City health & campaign changes

Although I've previously refused to add city health (citing that this is not a medieval game), I've realized that not having city health means key cities could be easily taken. I'm therefore adding optional city health that's available in the map editor, meaning cities need to have their health go to 0 before an enemy can occupy them. Attacking a garrisoned unit distributes part of the damage to the city health (and goes straight to the city health if there's no garrison). To reflect this update, some maps in the campaign (Siege of Leningrad, Stalingrad, Battle of Berlin, etc etc) now have fortified cities.