Needs to blow away old tiles

[chrislloyd]

Checkout https://minefold.com/worlds/minebnb/map. The size of the map has changed so the location of the tiles is different. I'm not too sure how to get around this. The easiest way for the moment would be to blow away old tiles before new ones are uploaded. Depending on how long the upload takes the map could be unavailable for a while. Maybe clear the mapped_at field and re-write it when it's finished uploading?