R1:0308d580d7df

Deactivate SSH keys instead of destroying them completely

Summary:
Ref T10917. Currently, when you delete an SSH key, we really truly delete it forever.

This isn't very consistent with other applications, but we built this stuff a long time ago before we were as rigorous about retaining data and making it auditable.

In partiular, destroying data isn't good for auditing after security issues, since it means we can't show you logs of any changes an attacker might have made to your keys.

To prepare to…