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.
Also, let's move this logic down to the model layer and have a `copyItem` just like we have `copyFile`. This will be useful for, e.g. plugin developers who want to reuse this logic from their own backend code.