Refactor map storage structure to flatten objects

[chrisella]

• Move all objects to a map root collection
• Keep layers collection as this can contain layer specific properties e.g. z-index, name, who can see etc
• Add new layerId prop to all MapObjects
• Update the saga update logic to work with new structure

Benefits:

• Make finding an object with an id in a map MUCH easier
• updates no longer need to target /map/layers/{layerid}/children/{objectid}, would now just be /map/objects/{objectId}
• Moving between layers becomes trivial, just update the layerId property