Allow overriding town settings on a per-town basis

Add a setting for whether this is allowed for non-privileged
multiplayer clients

src/command_type.h

@@ -347,6 +347,7 @@ enum Commands {
 	CMD_RENAME_TOWN,                  ///< rename a town
 	CMD_RENAME_TOWN_NON_ADMIN,        ///< rename a town, non-admin command
 	CMD_DO_TOWN_ACTION,               ///< do a action from the town detail window (like advertises or bribe)
+	CMD_TOWN_SETTING_OVERRIDE,        ///< override a town setting
 	CMD_TOWN_CARGO_GOAL,              ///< set the goal of a cargo for a town
 	CMD_TOWN_GROWTH_RATE,             ///< set the town growth rate
 	CMD_TOWN_RATING,                  ///< set rating of a company in a town