Allow instance prefabs to be an instance-less prefab

[Isadora]

Repro

1. Create a prefab A
2. Create an instance B from prefab A
3. Delete prefab A

Expected
Prefab B will become a prefab by itself. Prefab A will be deleted.

Actual
Prefab A will be deleted. Prefab B will become "corrupted" and will need to be manually deleted. Any world instance of prefab A will be deleted from the world.

Fix
The fix is the tricky part. I guess we should scan all possible instances of prefab A prior to deleting it? Do we want to delete all the instances from the world or make them a complete instance (no prefab attached)?

We should come back at this eventually.