Grains dropped from collection if unlinked/renamed

I think this may be related to #8 also but is a more severe issue.

Let's say user A shares a collection with user B

User B may try to rename a grain by unlinking it from a collection
and then re-linking it.

This results in them no longer having access to the grain,
they need to request access again.

This seems problematic as the intention of user A was to share the grain.