duplicate clients can be created

Because of the way the node title is constructed for clients, it's perfectly possible and easy for users to create two clients with the same apparent name even if they are unique (unicity is currently based on email for clients).

This is particularly confusing when creating a site for a client or when selecting a client in any form: you get only one client back from the autocomplete, but it can be the wrong one. In that case we end up in a weird SNAFU: we choose a client we were presented, but we can't actually create a site with it because we don't have the perms. Really weird.

I think we should completely refactor the fields in the Client, but maybe that's not necessary to fix this issue, so I'll file that as a separate request (#962330: refactor and clarify the fields in the client content type).