User can import published resources as private resources in their own "library"

[eethann]

Problem: if we allow users to "unpublish" resources, or allow shared resources to be mutable, then workbooks that rely on resources that have been unpublished might break.

Proposed solution: allow users to "import" (either manually or automatically) published resources into their own account when they use them.

This might result in a proliferation of copied data, so should be re-evaluated on implementation.

Another solution to the problem is to not allow users to unpublish published resources.