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.
Won't this also copy the `_id` field from srcFile, making it override it when we save? Let's either `del file['_id']` or manually copy over just the fields we want.