Add config for referencing Assets using model ids instead of container::path ids

[ryanmitchell]

This PR provides a config option to reference assets by their model id, instead of their path, so in your entry data they would be stored as container_handle::model_id instead of container_handle::path_to_file.

The benefit of this approach is that when you move the file to a different location any asset references don't need to be updated, potentially removing the need for a lot of updates.

• Write tests
• Add update command
• Work out how to prevent UpdateAssetReferences from running on save