Add Treasure-hunt-compatible Mob spawners

Add some more mob spawners with headgear that can live outside; also
silverfish swarm.  Change landmark_mobs to include these new spawners
and to remove the ones without headgear.
Added Herobrine spawner

Because my kids want to meet him in the game.
This Herobrine is a bit overpowered, but I guess that's the idea.