Item copy

This implements an item copy rest call via /item/{} POST.  It adds test coverage for the new functions.  As part of this, I've added a method to the files model to copy a file.  For all of our current three asset stores, I've added code to allow multiple files to use the same data through the use of a repeated reference.
We have an open issue for adding a client-side test for the S3 transfers, but that's out of scope of this topic. For testing the delete, moto will allow us to mock that one request to give a response as though the key exists.