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.

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