Allow revoking share access

[jpsim]

Sharing support was added in #191. It would be useful to also be able to revoke share access. Here's one way it could be built:

I just think about one more thing which is "unsharing", are we going to support it also? Actually it should be easy to implement this by not deleting ShareOffer/ShareRequest but keeping it and delete them only in case of "unsharing" with the associated ListReference.

That's a good idea, and could be built on top of this. If we add a settings level (#181) to the navigation hierarchy, it could list who has access to lists and you could revoke sharing there.