[teams] Teams can be deleted, even if there are resources which are shared through them

There is, currently, some inconsistence with the team deletion property and sharing.

If a contribution is shared via a team with someone, then the contribution is marked as "Shared" (sharing status) and the team is add to its shared_with_team list.

The owner of the team may now proceed and delete the team, which will leave the contribution marked with Shared, but now with no one on the shared_with_team list. If the user has used the contribution, I'm not sure in which status that leaves us.

My understanding for the simplest way out is this: if a team has been used to share a contribution, it can no longer be deleted.