Record spawn point

[chrislloyd]

We need to record the location of the spawn point when we map a world so we can focus the viewport. I'm thinking:

world: {
  id: '...'
  markers: [
    {
      _id: BSON id
      _type: "SpawnMarker"
      x: 1
      y: 2
      z: 3
    }
    {...}
  ]
}