Enable cross-modulestore copying of course asset metadata.

https://openedx.atlassian.net/browse/PLAT-38
I'd really like to figure out when interfaces should raise this error v just passing. If the impls all do what's in their scope and call super(), then it will raise this error and calling super() is a pattern I strongly support for any impl that's not absolutely certain that it did everything that any application could want. afaik, if the method is marked abstract, it will automatically raise this exception if it's called even if its body is `pass` but, I believe, super() will not call the abstract methods and thus not raise the exception. I should just rtfm and ensure my understanding is correct.