Implement Asset.ref for name or URI references

[uranusjr]

This allows us to refer to an asset without needing the original object, making it easier to schedule against an asset.

I also added some minor improvements to register_asset_change so we can save a db query or the asset-model conversion in some cases.

There are a couple of possible sytax designs available:

# Magic constructor.
AssetRef(name="asset-1")
AssetRef(uri="s3://bucket/assets/1")

# Magic factory method on Asset.
Asset.ref(name="asset-1")
Asset.ref(uri="s3://bucket/assets/1")

I’m opting for the second one with a factory method since it is easier to implement, but we can discuss which one we like better.