Ensure we do not delete actors with users to avoid impersonation
When we delete actors/users from the database, we're not keeping a track of the old object. This means that a user can potentially resignup with the same username, and impersonate the previous user.
One thing we can do is keep a track of deleted usernames in a separate table (when we implement #712 (closed) and #364 (closed)) so that it's not possible to reuse those names.
However, it also has the implication that people can block the usage of a name forever on any given server. Is it okay?