Replace final chest in treasure hunt with Shulker Box when Locked

[JiFish]

Replace final chest in treasure hunt with Shulker Box when Locked. To prevent players in survival mode mining the chest to loot it.

[sshipway]

This should also be done to any intermediate book chests (when the clues are split over multiple stages and books/keys)

[sshipway]

I now have this working; just need to push it up. I needed to add some shulker_box support to the tile_entity functions in utils.py to support this (a shulker box with a chest tile entity behaves oddly); however all now seems to work, including the locking.

[sshipway]

Fixed in merge #431