Add #6887: Option to show zone inside local authority boundary of towns

Can be found at town information > local authority window
Layout for button is same as Graph Keys
Turn on/off for every town individually

src/town_cmd.cpp

@@ -1746,6 +1746,7 @@ static void DoCreateTown(Town *t, TileIndex tile, uint32 townnameparts, TownSize
 	 * similar towns they're unlikely to grow all in one tick */
 	t->grow_counter = t->index % TOWN_GROWTH_TICKS;
 	t->growth_rate = TownTicksToGameTicks(250);
+	t->show_zone = false;
 
 	_town_kdtree.Insert(t->index);