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

[macaws]

This could greatly help on a project we've been working on - hundreds of thousand of assets.

Anything I can do to help here @ryanmitchell?

[ryanmitchell]

It’s waiting for @jasonvarga to give me his thoughts. It was his idea.

[ryanmitchell]

@jasonvarga I've tidied up the outstanding issues with this and its ready for a once over if you have time. Let me know if anything needs changed.