Implement Asset garbage collection

[mvandenburgh]

This PR implements asset garbage collection. See #2367 for the design doc/further explanation.

Leaving this as draft for now as it is contingent on consensus for #2367.

TODO:

• Add tests

[jjnesbitt]

Is there any downside to doing the following instead? This achieves the same effect using a JOIN, which is simpler if nothing else.

Suggested change

	return Asset.objects.alias(
	has_version=Exists(
	Version.objects.filter(
	assets=OuterRef('id'),
	),
	)
	).filter(
	has_version=False,
	published=False,
	blob__isnull=False, # only delete assets with blobs; zarrs are not supported yet
	modified__lt=timezone.now() - ASSET_EXPIRATION_TIME,
	)
	return Asset.objects.filter(
	versions__isnull=True,
	published=False,
	blob__isnull=False, # only delete assets with blobs; zarrs are not supported yet
	modified__lt=timezone.now() - ASSET_EXPIRATION_TIME,
	)